Rack User Locale

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A Rack module for getting and setting a user's locale via a cookie or browser default language.


  • Ruby >= 3.0

For Ruby < 3.0 use version 2.0.1.


In a Gemfile:

gem 'rack-user-locale', require: "user_locale"


Add the following line to your Rack application

use Rack::UserLocale

and thats it!!

Your application will now attempt to set the I18n.locale value based on the following:

  • Whether a user has a user-locale cookie set
  • The "HTTP_ACCEPT_LANGUAGE" value. If there are multiple values it will attempt to set the one ranked highest
  • The I18n.default_locale value (basically a fallback)


The user locale detection is done on every Rack::Request, which would include all of the non Ruby/Rack actions like .css or .js files.

If you don't want this happen then you can use Rack::Static in your application and filter out which files Rack should serve up statically.

Something like this:

use Rack::Static, urls: ["/css", "/js"], root: "public"

This would serve anything in your public folder statically and wouldn't create a Rack::Request.

See the Rack::Static docs for more info.


Accepted Locales

use Rack::UserLocale, accepted_locales: [:de, :en, :es, :fr, :ja, :pt, :zh] #(whatever codes you support)

If this option is supplied the users locale will either be set to one of the accepted locales in the array, otherwise it will be set to the I18n.default_locale value.

Should you wish to overwrite a users locale value at any point in your application, like for changing language prefs, then simple rewrite the user-locale cookie with a new value.

If you don't want to use the default cookie name of user-locale, you can set it to something else by passing a cookie_name option.

use Rack::UserLocale, cookie_name: "some-cookie-name"


Credit to these gems for pointing me in the direction I wanted to go in...


Bug reports and pull requests are welcome on GitHub at https://github.com/technicalpanda/rack-user-locale. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.


The gem is available as open source under the terms of the MIT License.

Code of Conduct

Everyone interacting with this project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.