Internationalization is a complex problem. Natural languages differ in so many ways (eg. 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:

As part of this solution, every static string in the Rails framework — eg. ActiveRecord validation messages, time and date formats — has been internationalized, so localization of a Rails application means "over-riding" these defaults.

Thus, the Ruby I18n gem is split into two parts:

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.

The most important methods of the I18n API are:

These have the aliases #t and #l so you can use them like this:

There are also attribute readers and writers for the following attributes:

So, let’s internationalize a simple Rails application from the ground up in the next chapters!

There are just a few, simple steps to get up and running with I18n support for your application.

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 config/locales directory to your translations load path, automatically.

See the default en.yml locale in this directory, containing a sample pair of translation strings:

This means, that in the :en locale, the key hello will map to Hello world string. Every string inside Rails is internationalized in this way, see for instance ActiveRecord validation messages in the activerecord/lib/active_record/locale/en.yml file or time and date formats in the 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, ie. if you don’t set a different locale, :en will be used for looking up translations.

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.

The default environment.rb files has instruction how to add locales from another directory and how to set different default locale. Just uncomment and edit the specific lines.

For the sake of completeness, let’s mention that if you do not want to use the environment.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:

By default the I18n library will use :en (English) as a I18n.default_locale for looking up translations (if you do not specify a locale for a lookup).

If you want to translate your Rails application to a single language other than English you can set I18n.default_locale to your locale. If you want to change the locale on a per-request basis though you can set it in a before_filter on the ApplicationController like this:

This will already work for URLs where you pass the locale as a query parameter as in example.com?locale=pt (which is what Google also does).

Now you’ve initialized I18n support for your application and told it which locale should be used. With that in place you’re now ready for the really interesting stuff.

The process of "internationalization" usually means to abstract all strings and other locale specific bits out of your application. The process of "localization" means to then provide translations and localized formats for these bits. [1]

So, let’s internationalize something. You most probably have something like this in one of your applications:

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:

When you now render this view it will show an error message that tells you that the translations for the keys :hello_world and :hello_flash are missing.

So let’s add the missing translations (i.e. do the "localization" part):

There you go. Because you haven’t changed the default_locale I18n will use English. Your application now shows:

And when you change the URL to pass the pirate locale you get:

NOTE You need to restart the server when you add new locale files.

Ok, 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.

And in our pirate translations file let’s add a time format (it’s already there in Rails' defaults for English):

So that would give you:

NOTE Right now you might need to add some more date/time formats in order to make the I18n backend work as expected. See the rails-i18n repository for starting points.

The following purposes are covered:

Translations are looked up by keys which can be both Symbols or Strings, so these calls are equivalent:

translate also takes a :scope option which can contain one or many additional keys that will be used to specify a “namespace” or scope for a translation key:

This looks up the :invalid message in the ActiveRecord error messages.

Additionally, both the key and scopes can be specified as dot separated keys as in:

Thus the following calls are equivalent:

When a default option is given its value will be returned if the translation is missing:

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:

To lookup multiple translations at once an array of keys can be passed:

Also, a key can translate to a (potentially nested) hash as grouped translations. E.g. one can receive all ActiveRecord error messages as a Hash with:

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:

If a translation uses :default or :scope as a interpolation variable an I18n::ReservedInterpolationKey exception is raised. If a translation expects an interpolation variable but it has not been passed to #translate an I18n::MissingInterpolationArgument exception is raised.

In English there’s only a singular and a plural form for a given string, e.g. "1 message" and "2 messages". Other languages (Arabic, Japanese, Russian and many more) have different grammars that have additional or less plural forms. 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:

The algorithm for pluralizations in :en is as simple as:

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 an Hash suitable for pluralization an I18n::InvalidPluralizationData exception is raised.

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:

Explicitely passing a locale:

I18n.locale defaults to I18n.default_locale which defaults to :en. The default locale can be set like this:

The shipped Simple backend allows you to store translations in both plain Ruby and YAML format. [2]

For example a Ruby Hash providing translations can look like this:

The equivalent YAML file would look like this:

As you see in both cases the toplevel key is the locale. :foo is a namespace key and :bar is the key for the translation "baz".

Here is a "real" example from the ActiveSupport en.yml translations YAML file:

So, all of the following equivalent lookups will return the :short date format "%B %d":

Generally we recommend using YAML as a format for storing translations. There are cases though where you want to store Ruby lambdas as part of your locale data, e.g. for special date

You can use the methods Model.human_name and Model.human_attribute_name(attribute) to transparently lookup translations for your model and attribute names.

For example when you add the following translations:

Then User.human_name will return "Dude" and User.human_attribute_name(:login) will return "Handle".

ActiveRecord validation error messages can also be translated easily. ActiveRecord gives you a couple of namespaces where you can place your message translations in order to provide different messages and translation for certain models, attributes and/or validations. It also transparently takes single table inheritance into account.

This gives you quite powerful means to flexibly adjust your messages to your application’s needs.

Consider a User model with a validates_presence_of validation for the name attribute like this:

The key for the error message in this case is :blank. ActiveRecord will lookup this key in the namespaces:

Thus, in our example it will try the following keys in this order and return the first result:

When your models are additionally using inheritance then the messages are looked up for the inherited model class names are looked up.

For example, you might have an Admin model inheriting from User:

Then ActiveRecord will look for messages in this order:

This way you can provide special translations for various error messages at different points in your models inheritance chain and in the attributes, models or default scopes.

The translated model name and translated attribute name are always available for interpolation.

count and/or value are available where applicable. Count can be used for pluralization if present:

If you are using the ActiveRecord error_messages_for helper you will want to add translations for it.

Rails ships with the following translations:

Rails uses fixed strings and other localizations, such as format strings and other format information in a couple of helpers.

TODO list helpers and available keys

For several reasons the shipped Simple backend only does the "simplest thing that ever could work" for Ruby on Rails [3] ... which means that it is only guaranteed to work for English and, as a side effect, languages that are very similar to English. Also, the simple backend is only capable of reading translations but can not dynamically store them to any format.

That does not mean you’re stuck with these limitations though. The Ruby I18n gem makes it very easy to exchange the Simple backend implementation with something else that fits better for your needs. E.g. you could exchange it with Globalize’s Static backend:

The I18n API defines the following exceptions that will be raised by backends when the corresponding unexpected conditions occur:

The I18n API will catch all of these exceptions when they were thrown in the backend and pass them to the default_exception_handler method. This method will re-raise all exceptions except for MissingTranslationData exceptions. When a MissingTranslationData exception has been caught it will return the exception’s error message string containing the missing key/scope.

The reason for this is that during development you’d usually want your views to still render even though a translation is missing.

In other contexts you might want to change this behaviour though. E.g. the default exception handling does not allow to catch missing translations during automated tests easily. For this purpose a different exception handler can be specified. The specified exception handler must be a method on the I18n module:

This would re-raise all caught exceptions including MissingTranslationData.

Another example where the default behaviour is less desirable is the Rails TranslationHelper which provides the method #t (as well as #translate). When a MissingTranslationData exception occurs in this context the helper wraps the message into a span with the css class translation_missing.

To do so the helper forces I18n#translate to raise exceptions no matter what exception handler is defined by setting the :raise option:

[1] Or, to quote Wikipedia: "Internationalization is the process of designing a software application so that it can be adapted to various languages and regions without engineering changes. Localization is the process of adapting software for a specific region or language by adding locale-specific components and translating text."

[2] Other backends might allow or require to use other formats, e.g. a GetText backend might allow to read GetText files.

[3] One of these reasons is that we don’t want to any unnecessary load for applications that do not need any I18n capabilities, so we need to keep the I18n library as simple as possible for English. Another reason is that it is virtually impossible to implement a one-fits-all solution for all problems related to I18n for all existing languages. So a solution that allows us to exchange the entire implementation easily is appropriate anyway. This also makes it much easier to experiment with custom features and extensions.

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