Rails Internationalization (I18n) API ===================================== The Ruby I18n (shorthand for _internationalization_) gem which is shipped with Ruby on Rails (starting from Rails 2.2) provides an easy-to-use and extensible framework for **translating your application to a single custom language** other than English or for **providing multi-language support** in your application. The process of "internationalization" usually means to abstract all strings and other locale specific bits (such as date or currency formats) out of your application. The process of "localization" means to provide translations and localized formats for these bits.[^1] So, in the process of _internationalizing_ your Rails application you have to: * Ensure you have support for i18n. * Tell Rails where to find locale dictionaries. * Tell Rails how to set, preserve and switch locales. In the process of _localizing_ your application you'll probably want to do the following three things: * Replace or supplement Rails' default locale — e.g. date and time formats, month names, Active Record model names, etc. * Abstract strings in your application into keyed dictionaries — e.g. flash messages, static text in your views, etc. * Store the resulting dictionaries somewhere. This guide will walk you through the I18n API and contains a tutorial on how to internationalize a Rails application from the start. -------------------------------------------------------------------------------- NOTE: The Ruby I18n framework provides you with all necessary means for internationalization/localization of your Rails application. You may, however, use any of various plugins and extensions available, which add additional functionality or features. See the Rails [I18n Wiki](http://rails-i18n.org/wiki) for more information. How I18n in Ruby on Rails Works ------------------------------- Internationalization is a complex problem. Natural languages differ in so many ways (e.g. in pluralization rules) that it is hard to provide tools for solving all problems at once. For that reason the Rails I18n API focuses on: * providing support for English and similar languages out of the box * making it easy to customize and extend everything for other languages As part of this solution, **every static string in the Rails framework** — e.g. Active Record validation messages, time and date formats — **has been internationalized**, so _localization_ of a Rails application means "over-riding" these defaults. ### The Overall Architecture of the Library Thus, the Ruby I18n gem is split into two parts: * The public API of the i18n framework — a Ruby module with public methods that define how the library works * A default backend (which is intentionally named _Simple_ backend) that implements these methods As a user you should always only access the public methods on the I18n module, but it is useful to know about the capabilities of the backend. NOTE: It is possible (or even desirable) to swap the shipped Simple backend with a more powerful one, which would store translation data in a relational database, GetText dictionary, or similar. See section [Using different backends](#using-different-backends) below. ### The Public I18n API The most important methods of the I18n API are: ```ruby translate # Lookup text translations localize # Localize Date and Time objects to local formats ``` These have the aliases #t and #l so you can use them like this: ```ruby I18n.t 'store.title' I18n.l Time.now ``` There are also attribute readers and writers for the following attributes: ```ruby load_path # Announce your custom translation files locale # Get and set the current locale default_locale # Get and set the default locale exception_handler # Use a different exception_handler backend # Use a different backend ``` So, let's internationalize a simple Rails application from the ground up in the next chapters! Setup the Rails Application for Internationalization ---------------------------------------------------- There are just a few simple steps to get up and running with I18n support for your application. ### Configure the I18n Module Following the _convention over configuration_ philosophy, Rails will set up your application with reasonable defaults. If you need different settings, you can overwrite them easily. Rails adds all `.rb` and `.yml` files from the `config/locales` directory to your **translations load path**, automatically. The default `en.yml` locale in this directory contains a sample pair of translation strings: ```ruby en: hello: "Hello world" ``` This means, that in the `:en` locale, the key _hello_ will map to the _Hello world_ string. Every string inside Rails is internationalized in this way, see for instance Active Record validation messages in the [`activerecord/lib/active_record/locale/en.yml`](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/locale/en.yml file or time and date formats in the [`activesupport/lib/active_support/locale/en.yml`](https://github.com/rails/rails/blob/master/activesupport/lib/active_support/locale/en.yml) file. You can use YAML or standard Ruby Hashes to store translations in the default (Simple) backend. The I18n library will use **English** as a **default locale**, i.e. if you don't set a different locale, `:en` will be used for looking up translations. NOTE: The i18n library takes a **pragmatic approach** to locale keys (after [some discussion](http://groups.google.com/group/rails-i18n/browse_thread/thread/14dede2c7dbe9470/80eec34395f64f3c?hl=en), including only the _locale_ ("language") part, like `:en`, `:pl`, not the _region_ part, like `:en-US` or `:en-GB`, which are traditionally used for separating "languages" and "regional setting" or "dialects". Many international applications use only the "language" element of a locale such as `:cs`, `:th` or `:es` (for Czech, Thai and Spanish). However, there are also regional differences within different language groups that may be important. For instance, in the `:en-US` locale you would have $ as a currency symbol, while in `:en-GB`, you would have £. Nothing stops you from separating regional and other settings in this way: you just have to provide full "English - United Kingdom" locale in a `:en-GB` dictionary. Various [Rails I18n plugins](http://rails-i18n.org/wiki) such as [Globalize2](https://github.com/joshmh/globalize2/tree/master) may help you implement it. The **translations load path** (`I18n.load_path`) is just a Ruby Array of paths to your translation files that will be loaded automatically and available in your application. You can pick whatever directory and translation file naming scheme makes sense for you. NOTE: The backend will lazy-load these translations when a translation is looked up for the first time. This makes it possible to just swap the backend with something else even after translations have already been announced. The default initializer `locale.rb` file has instructions on how to add locales from another directory and how to set a different default locale. Just uncomment and edit the specific lines. ```ruby # The default locale is :en and all translations from config/locales/*.rb,yml are auto loaded. # config.i18n.load_path += Dir[Rails.root.join('my', 'locales', '*.{rb,yml}').to_s] # config.i18n.default_locale = :de ``` ### Optional: Custom I18n Configuration Setup For the sake of completeness, let's mention that if you do not want to use the `application.rb` file for some reason, you can always wire up things manually, too. To tell the I18n library where it can find your custom translation files you can specify the load path anywhere in your application - just make sure it gets run before any translations are actually looked up. You might also want to change the default locale. The simplest thing possible is to put the following into an initializer: ```ruby # in config/initializers/locale.rb # tell the I18n library where to find your translations I18n.load_path += Dir[Rails.root.join('lib', 'locale', '*.{rb,yml}')] # set default locale to something other than :en I18n.default_locale = :pt ``` ### Setting and Passing the Locale If you want to translate your Rails application to a **single language other than English** (the default locale), you can set I18n.default_locale to your locale in `application.rb` or an initializer as shown above, and it will persist through the requests. However, you would probably like to **provide support for more locales** in your application. In such case, you need to set and pass the locale between requests. WARNING: You may be tempted to store the chosen locale in a _session_ or a cookie, however **do not do this**. The locale should be transparent and a part of the URL. This way you won't break people's basic assumptions about the web itself: if you send a URL to a friend, they should see the same page and content as you. A fancy word for this would be that you're being [RESTful](http://en.wikipedia.org/wiki/Representational_State_Transfer. Read more about the RESTful approach in [Stefan Tilkov's articles](http://www.infoq.com/articles/rest-introduction). Sometimes there are exceptions to this rule and those are discussed below. The _setting part_ is easy. You can set the locale in a `before_filter` in the `ApplicationController` like this: ```ruby before_filter :set_locale def set_locale I18n.locale = params[:locale] || I18n.default_locale end ``` This requires you to pass the locale as a URL query parameter as in `http://example.com/books?locale=pt`. (This is, for example, Google's approach.) So `http://localhost:3000?locale=pt` will load the Portuguese localization, whereas `http://localhost:3000?locale=de` would load the German localization, and so on. You may skip the next section and head over to the **Internationalize your application** section, if you want to try things out by manually placing the locale in the URL and reloading the page. Of course, you probably don't want to manually include the locale in every URL all over your application, or want the URLs look differently, e.g. the usual `http://example.com/pt/books` versus `http://example.com/en/books`. Let's discuss the different options you have. ### Setting the Locale from the Domain Name One option you have is to set the locale from the domain name where your application runs. For example, we want `www.example.com` to load the English (or default) locale, and `www.example.es` to load the Spanish locale. Thus the _top-level domain name_ is used for locale setting. This has several advantages: * The locale is an _obvious_ part of the URL. * People intuitively grasp in which language the content will be displayed. * It is very trivial to implement in Rails. * Search engines seem to like that content in different languages lives at different, inter-linked domains. You can implement it like this in your `ApplicationController`: ```ruby before_filter :set_locale def set_locale I18n.locale = extract_locale_from_tld || I18n.default_locale end # Get locale from top-level domain or return nil if such locale is not available # You have to put something like: # 127.0.0.1 application.com # 127.0.0.1 application.it # 127.0.0.1 application.pl # in your /etc/hosts file to try this out locally def extract_locale_from_tld parsed_locale = request.host.split('.').last I18n.available_locales.include?(parsed_locale.to_sym) ? parsed_locale : nil end ``` We can also set the locale from the _subdomain_ in a very similar way: ```ruby # Get locale code from request subdomain (like http://it.application.local:3000) # You have to put something like: # 127.0.0.1 gr.application.local # in your /etc/hosts file to try this out locally def extract_locale_from_subdomain parsed_locale = request.subdomains.first I18n.available_locales.include?(parsed_locale.to_sym) ? parsed_locale : nil end ``` If your application includes a locale switching menu, you would then have something like this in it: ```ruby link_to("Deutsch", "#{APP_CONFIG[:deutsch_website_url]}#{request.env['REQUEST_URI']}") ``` assuming you would set `APP_CONFIG[:deutsch_website_url]` to some value like `http://www.application.de`. This solution has aforementioned advantages, however, you may not be able or may not want to provide different localizations ("language versions") on different domains. The most obvious solution would be to include locale code in the URL params (or request path). ### Setting the Locale from the URL Params The most usual way of setting (and passing) the locale would be to include it in URL params, as we did in the `I18n.locale = params[:locale]` _before_filter_ in the first example. We would like to have URLs like `www.example.com/books?locale=ja` or `www.example.com/ja/books` in this case. This approach has almost the same set of advantages as setting the locale from the domain name: namely that it's RESTful and in accord with the rest of the World Wide Web. It does require a little bit more work to implement, though. Getting the locale from `params` and setting it accordingly is not hard; including it in every URL and thus **passing it through the requests** is. To include an explicit option in every URL (e.g. `link_to( books_url(locale: I18n.locale))`) would be tedious and probably impossible, of course. Rails contains infrastructure for "centralizing dynamic decisions about the URLs" in its [`ApplicationController#default_url_options`](http://api.rubyonrails.org/classes/ActionController/Base.html#M000515, which is useful precisely in this scenario: it enables us to set "defaults" for [`url_for`](http://api.rubyonrails.org/classes/ActionController/Base.html#M000503) and helper methods dependent on it (by implementing/overriding this method). We can include something like this in our `ApplicationController` then: ```ruby # app/controllers/application_controller.rb def default_url_options(options={}) logger.debug "default_url_options is passed options: #{options.inspect}\n" { locale: I18n.locale } end ``` Every helper method dependent on `url_for` (e.g. helpers for named routes like `root_path` or `root_url`, resource routes like `books_path` or `books_url`, etc.) will now **automatically include the locale in the query string**, like this: `http://localhost:3001/?locale=ja`. You may be satisfied with this. It does impact the readability of URLs, though, when the locale "hangs" at the end of every URL in your application. Moreover, from the architectural standpoint, locale is usually hierarchically above the other parts of the application domain: and URLs should reflect this. You probably want URLs to look like this: `www.example.com/en/books` (which loads the English locale) and `www.example.com/nl/books` (which loads the Dutch locale). This is achievable with the "over-riding `default_url_options`" strategy from above: you just have to set up your routes with [`scoping`](http://api.rubyonrails.org/classes/ActionDispatch/Routing/Mapper/Scoping.html) option in this way: ```ruby # config/routes.rb scope "/:locale" do resources :books end ``` Now, when you call the `books_path` method you should get `"/en/books"` (for the default locale). An URL like `http://localhost:3001/nl/books` should load the Dutch locale, then, and following calls to `books_path` should return `"/nl/books"` (because the locale changed). If you don't want to force the use of a locale in your routes you can use an optional path scope (denoted by the parentheses) like so: ```ruby # config/routes.rb scope "(:locale)", locale: /en|nl/ do resources :books end ``` With this approach you will not get a `Routing Error` when accessing your resources such as `http://localhost:3001/books` without a locale. This is useful for when you want to use the default locale when one is not specified. Of course, you need to take special care of the root URL (usually "homepage" or "dashboard") of your application. An URL like `http://localhost:3001/nl` will not work automatically, because the `root to: "books#index"` declaration in your `routes.rb` doesn't take locale into account. (And rightly so: there's only one "root" URL.) You would probably need to map URLs like these: ```ruby # config/routes.rb match '/:locale' => 'dashboard#index' ``` Do take special care about the **order of your routes**, so this route declaration does not "eat" other ones. (You may want to add it directly before the `root :to` declaration.) NOTE: Have a look at two plugins which simplify work with routes in this way: Sven Fuchs's [routing_filter](https://github.com/svenfuchs/routing-filter/tree/master and Raul Murciano's [translate_routes](https://github.com/raul/translate_routes/tree/master). ### Setting the Locale from the Client Supplied Information In specific cases, it would make sense to set the locale from client-supplied information, i.e. not from the URL. This information may come for example from the users' preferred language (set in their browser), can be based on the users' geographical location inferred from their IP, or users can provide it simply by choosing the locale in your application interface and saving it to their profile. This approach is more suitable for web-based applications or services, not for websites — see the box about _sessions_, _cookies_ and RESTful architecture above. #### Using `Accept-Language` One source of client supplied information would be an `Accept-Language` HTTP header. People may [set this in their browser](http://www.w3.org/International/questions/qa-lang-priorities) or other clients (such as _curl_). A trivial implementation of using an `Accept-Language` header would be: ```ruby def set_locale logger.debug "* Accept-Language: #{request.env['HTTP_ACCEPT_LANGUAGE']}" I18n.locale = extract_locale_from_accept_language_header logger.debug "* Locale set to '#{I18n.locale}'" end private def extract_locale_from_accept_language_header request.env['HTTP_ACCEPT_LANGUAGE'].scan(/^[a-z]{2}/).first end ``` Of course, in a production environment you would need much more robust code, and could use a plugin such as Iain Hecker's [http_accept_language](https://github.com/iain/http_accept_language/tree/master or even Rack middleware such as Ryan Tomayko's [locale](https://github.com/rack/rack-contrib/blob/master/lib/rack/contrib/locale.rb). #### Using GeoIP (or Similar) Database Another way of choosing the locale from client information would be to use a database for mapping the client IP to the region, such as [GeoIP Lite Country](http://www.maxmind.com/app/geolitecountry). The mechanics of the code would be very similar to the code above — you would need to query the database for the user's IP, and look up your preferred locale for the country/region/city returned. #### User Profile You can also provide users of your application with means to set (and possibly over-ride) the locale in your application interface, as well. Again, mechanics for this approach would be very similar to the code above — you'd probably let users choose a locale from a dropdown list and save it to their profile in the database. Then you'd set the locale to this value. Internationalizing your Application ----------------------------------- OK! Now you've initialized I18n support for your Ruby on Rails application and told it which locale to use and how to preserve it between requests. With that in place, you're now ready for the really interesting stuff. Let's _internationalize_ our application, i.e. abstract every locale-specific parts, and then _localize_ it, i.e. provide necessary translations for these abstracts. You most probably have something like this in one of your applications: ```ruby # config/routes.rb Yourapp::Application.routes.draw do root to: "home#index" end ``` ```ruby # app/controllers/home_controller.rb class HomeController < ApplicationController def index flash[:notice] = "Hello Flash" end end ``` ```html+erb # app/views/home/index.html.erb
<%= flash[:notice] %>
``` ![rails i18n demo untranslated](images/i18n/demo_untranslated.png) ### Adding Translations Obviously there are **two strings that are localized to English**. In order to internationalize this code, **replace these strings** with calls to Rails' `#t` helper with a key that makes sense for the translation: ```ruby # app/controllers/home_controller.rb class HomeController < ApplicationController def index flash[:notice] = t(:hello_flash) end end ``` ```html+erb # app/views/home/index.html.erb<%= flash[:notice] %>
``` When you now render this view, it will show an error message which tells you that the translations for the keys `:hello_world` and `:hello_flash` are missing. ![rails i18n demo translation missing](images/i18n/demo_translation_missing.png) NOTE: Rails adds a `t` (`translate`) helper method to your views so that you do not need to spell out `I18n.t` all the time. Additionally this helper will catch missing translations and wrap the resulting error message into a ``. So let's add the missing translations into the dictionary files (i.e. do the "localization" part): ```ruby # config/locales/en.yml en: hello_world: Hello world! hello_flash: Hello flash! # config/locales/pirate.yml pirate: hello_world: Ahoy World hello_flash: Ahoy Flash ``` There you go. Because you haven't changed the default_locale, I18n will use English. Your application now shows: ![rails i18n demo translated to English](images/i18n/demo_translated_en.png) And when you change the URL to pass the pirate locale (`http://localhost:3000?locale=pirate`), you'll get: ![rails i18n demo translated to pirate](images/i18n/demo_translated_pirate.png) NOTE: You need to restart the server when you add new locale files. You may use YAML (`.yml`) or plain Ruby (`.rb`) files for storing your translations in SimpleStore. YAML is the preferred option among Rails developers. However, it has one big disadvantage. YAML is very sensitive to whitespace and special characters, so the application may not load your dictionary properly. Ruby files will crash your application on first request, so you may easily find what's wrong. (If you encounter any "weird issues" with YAML dictionaries, try putting the relevant portion of your dictionary into a Ruby file.) ### Passing variables to translations You can use variables in the translation messages and pass their values from the view. ```erb # app/views/home/index.html.erb <%=t 'greet_username', user: "Bill", message: "Goodbye" %> ``` ```yaml # config/locales/en.yml en: greet_username: "%{message}, %{user}!" ``` ### Adding Date/Time Formats OK! Now let's add a timestamp to the view, so we can demo the **date/time localization** feature as well. To localize the time format you pass the Time object to `I18n.l` or (preferably) use Rails' `#l` helper. You can pick a format by passing the `:format` option — by default the `:default` format is used. ```erb # app/views/home/index.html.erb<%= flash[:notice] %>
<%= l Time.now, format: :short %> ``` And in our pirate translations file let's add a time format (it's already there in Rails' defaults for English): ```ruby # config/locales/pirate.yml pirate: time: formats: short: "arrrround %H'ish" ``` So that would give you: ![rails i18n demo localized time to pirate](images/i18n/demo_localized_pirate.png) TIP: Right now you might need to add some more date/time formats in order to make the I18n backend work as expected (at least for the 'pirate' locale). Of course, there's a great chance that somebody already did all the work by **translating Rails' defaults for your locale**. See the [rails-i18n repository at Github](https://github.com/svenfuchs/rails-i18n/tree/master/rails/locale) for an archive of various locale files. When you put such file(s) in `config/locales/` directory, they will automatically be ready for use. ### Inflection Rules For Other Locales Rails 4.0 allows you to define inflection rules (such as rules for singularization and pluralization) for locales other than English. In `config/initializers/inflections.rb`, you can define these rules for multiple locales. The initializer contains a default example for specifying additional rules for English; follow that format for other locales as you see fit. ### Localized Views Rails 2.3 introduces another convenient localization feature: localized views (templates). Let's say you have a _BooksController_ in your application. Your _index_ action renders content in `app/views/books/index.html.erb` template. When you put a _localized variant_ of this template: `index.es.html.erb` in the same directory, Rails will render content in this template, when the locale is set to `:es`. When the locale is set to the default locale, the generic `index.html.erb` view will be used. (Future Rails versions may well bring this _automagic_ localization to assets in `public`, etc.) You can make use of this feature, e.g. when working with a large amount of static content, which would be clumsy to put inside YAML or Ruby dictionaries. Bear in mind, though, that any change you would like to do later to the template must be propagated to all of them. ### Organization of Locale Files When you are using the default SimpleStore shipped with the i18n library, dictionaries are stored in plain-text files on the disc. Putting translations for all parts of your application in one file per locale could be hard to manage. You can store these files in a hierarchy which makes sense to you. For example, your `config/locales` directory could look like this: ``` |-defaults |---es.rb |---en.rb |-models |---book |-----es.rb |-----en.rb |-views |---defaults |-----es.rb |-----en.rb |---books |-----es.rb |-----en.rb |---users |-----es.rb |-----en.rb |---navigation |-----es.rb |-----en.rb ``` This way, you can separate model and model attribute names from text inside views, and all of this from the "defaults" (e.g. date and time formats). Other stores for the i18n library could provide different means of such separation. NOTE: The default locale loading mechanism in Rails does not load locale files in nested dictionaries, like we have here. So, for this to work, we must explicitly tell Rails to look further: ```ruby # config/application.rb config.i18n.load_path += Dir[Rails.root.join('config', 'locales', '**', '*.{rb,yml}')] ``` Do check the [Rails i18n Wiki](http://rails-i18n.org/wiki) for list of tools available for managing translations. Overview of the I18n API Features --------------------------------- You should have good understanding of using the i18n library now, knowing all necessary aspects of internationalizing a basic Rails application. In the following chapters, we'll cover it's features in more depth. Covered are features like these: * looking up translations * interpolating data into translations * pluralizing translations * using safe HTML translations * localizing dates, numbers, currency, etc. ### Looking up Translations #### Basic Lookup, Scopes and Nested Keys Translations are looked up by keys which can be both Symbols or Strings, so these calls are equivalent: ```ruby I18n.t :message I18n.t 'message' ``` The `translate` method also takes a `:scope` option which can contain one or more additional keys that will be used to specify a “namespace” or scope for a translation key: ```ruby I18n.t :record_invalid, scope: [:activerecord, :errors, :messages] ``` This looks up the `:record_invalid` message in the Active Record error messages. Additionally, both the key and scopes can be specified as dot-separated keys as in: ```ruby I18n.translate "activerecord.errors.messages.record_invalid" ``` Thus the following calls are equivalent: ```ruby I18n.t 'activerecord.errors.messages.record_invalid' I18n.t 'errors.messages.record_invalid', scope: :active_record I18n.t :record_invalid, scope: 'activerecord.errors.messages' I18n.t :record_invalid, scope: [:activerecord, :errors, :messages] ``` #### Defaults When a `:default` option is given, its value will be returned if the translation is missing: ```ruby I18n.t :missing, default: 'Not here' # => 'Not here' ``` If the `:default` value is a Symbol, it will be used as a key and translated. One can provide multiple values as default. The first one that results in a value will be returned. E.g., the following first tries to translate the key `:missing` and then the key `:also_missing.` As both do not yield a result, the string "Not here" will be returned: ```ruby I18n.t :missing, default: [:also_missing, 'Not here'] # => 'Not here' ``` #### Bulk and Namespace Lookup To look up multiple translations at once, an array of keys can be passed: ```ruby I18n.t [:odd, :even], scope: 'errors.messages' # => ["must be odd", "must be even"] ``` Also, a key can translate to a (potentially nested) hash of grouped translations. E.g., one can receive _all_ Active Record error messages as a Hash with: ```ruby I18n.t 'activerecord.errors.messages' # => {:inclusion=>"is not included in the list", :exclusion=> ... } ``` #### "Lazy" Lookup Rails implements a convenient way to look up the locale inside _views_. When you have the following dictionary: ```yaml es: books: index: title: "Título" ``` you can look up the `books.index.title` value **inside** `app/views/books/index.html.erb` template like this (note the dot): ```erb <%= t '.title' %> ``` ### Interpolation In many cases you want to abstract your translations so that **variables can be interpolated into the translation**. For this reason the I18n API provides an interpolation feature. All options besides `:default` and `:scope` that are passed to `#translate` will be interpolated to the translation: ```ruby I18n.backend.store_translations :en, thanks: 'Thanks %{name}!' I18n.translate :thanks, name: 'Jeremy' # => 'Thanks Jeremy!' ``` If a translation uses `:default` or `:scope` as an interpolation variable, an `I18n::ReservedInterpolationKey` exception is raised. If a translation expects an interpolation variable, but this has not been passed to `#translate`, an `I18n::MissingInterpolationArgument` exception is raised. ### Pluralization In English there are only one singular and one plural form for a given string, e.g. "1 message" and "2 messages". Other languages ([Arabic](http://unicode.org/repos/cldr-tmp/trunk/diff/supplemental/language_plural_rules.html#ar), [Japanese](http://unicode.org/repos/cldr-tmp/trunk/diff/supplemental/language_plural_rules.html#ja), [Russian](http://unicode.org/repos/cldr-tmp/trunk/diff/supplemental/language_plural_rules.html#ru) and many more) have different grammars that have additional or fewer [plural forms](http://unicode.org/repos/cldr-tmp/trunk/diff/supplemental/language_plural_rules.html). Thus, the I18n API provides a flexible pluralization feature. The `:count` interpolation variable has a special role in that it both is interpolated to the translation and used to pick a pluralization from the translations according to the pluralization rules defined by CLDR: ```ruby I18n.backend.store_translations :en, inbox: { one: 'one message', other: '%{count} messages' } I18n.translate :inbox, count: 2 # => '2 messages' I18n.translate :inbox, count: 1 # => 'one message' ``` The algorithm for pluralizations in `:en` is as simple as: ```ruby entry[count == 1 ? 0 : 1] ``` I.e. the translation denoted as `:one` is regarded as singular, the other is used as plural (including the count being zero). If the lookup for the key does not return a Hash suitable for pluralization, an `18n::InvalidPluralizationData` exception is raised. ### Setting and Passing a Locale The locale can be either set pseudo-globally to `I18n.locale` (which uses `Thread.current` like, e.g., `Time.zone`) or can be passed as an option to `#translate` and `#localize`. If no locale is passed, `I18n.locale` is used: ```ruby I18n.locale = :de I18n.t :foo I18n.l Time.now ``` Explicitly passing a locale: ```ruby I18n.t :foo, locale: :de I18n.l Time.now, locale: :de ``` The `I18n.locale` defaults to `I18n.default_locale` which defaults to :`en`. The default locale can be set like this: ```ruby I18n.default_locale = :de ``` ### Using Safe HTML Translations Keys with a '_html' suffix and keys named 'html' are marked as HTML safe. Use them in views without escaping. ```yaml # config/locales/en.yml en: welcome: welcome! hello_html: hello! title: html: title! ``` ```html+erb # app/views/home/index.html.erb