Accept Language 🌐

A tiny library for parsing the Accept-Language header from browsers (as defined in RFC 2616).

Status

Why this tool?

Thread-safe implementation.
Small algorithm that can handle tricky cases.
Match strings and symbols ignoring the case.
Works also well without Rails, Rack, i18n.
Comes with BCP 47 support.

Installation

Add this line to your application's Gemfile:

gem "accept_language"

And then execute:

bundle install

Or install it yourself as:

gem install accept_language

Usage

It's intended to be used in a Web server that supports some level of internationalization (i18n), but can be used anytime an Accept-Language header string is available.

In order to help facilitate better i18n, the lib try to find the intersection of the languages the user prefers and the languages your application supports.

Some examples:

AcceptLanguage.parse("da, en-GB;q=0.8, en;q=0.7").match(:en, :da)       # => :da
AcceptLanguage.parse("da, en;q=0.8, ug;q=0.9").match("en-GB", "ug-CN")  # => "ug-CN"
AcceptLanguage.parse("da, en-GB;q=0.8, en;q=0.7").match(:ja)            # => nil
AcceptLanguage.parse("fr-CH").match(:fr)                                # => nil
AcceptLanguage.parse("de, zh;q=0.4, fr;q=0").match(:fr)                 # => nil
AcceptLanguage.parse("de, zh;q=0.4, *;q=0.5, fr;q=0").match(:ar)        # => :ar
AcceptLanguage.parse("uz-latn-uz").match("uz-Latn-UZ")                  # => "uz-Latn-UZ"
AcceptLanguage.parse("foo;q=0.1").match(:FoO)                           # => :FoO
AcceptLanguage.parse("foo").match("bar")                                # => nil
AcceptLanguage.parse("*").match("BaZ")                                  # => "BaZ"
AcceptLanguage.parse("*;q=0").match("foobar")                           # => nil
AcceptLanguage.parse("en, en;q=0").match("en")                          # => nil
AcceptLanguage.parse("*, en;q=0").match("en")                           # => nil

Rails integration example

# app/controllers/application_controller.rb
class ApplicationController < ActionController::Base
  before_action :best_locale_from_request!

  def best_locale_from_request!
    I18n.locale = best_locale_from_request
  end

  def best_locale_from_request
    return I18n.default_locale unless request.headers.key?("HTTP_ACCEPT_LANGUAGE")

    string = request.headers.fetch("HTTP_ACCEPT_LANGUAGE")
    locale = AcceptLanguage.parse(string).match(*I18n.available_locales)

    # If the server cannot serve any matching language,
    # it can theoretically send back a 406 (Not Acceptable) error code.
    # But, for a better user experience, this is rarely done and more
    # common way is to ignore the Accept-Language header in this case.
    return I18n.default_locale if locale.nil?

    locale
  end
end

Versioning

AcceptLanguage uses Semantic Versioning 2.0.0

License

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