Parse locale from Accept-Language HTTP header

Gemfile

@@ -7,6 +7,7 @@ gem 'dotenv-rails'
 gem 'jbuilder'
 gem 'pg'
 gem 'puma'
+gem 'rack-contrib'
 gem 'rack-cors', require: 'rack/cors'
 gem 'rails'
 

Gemfile.lock

@@ -116,6 +116,8 @@ GEM
       nio4r (~> 2.0)
     racc (1.5.2)
     rack (2.2.3)
+    rack-contrib (2.3.0)
+      rack (~> 2.0)
     rack-cors (1.1.1)
       rack (>= 2.0.0)
     rack-test (1.1.0)
@@ -207,6 +209,7 @@ DEPENDENCIES
   pg
   pry-rails
   puma
+  rack-contrib
   rack-cors
   rails
   rubocop

config/application.rb

@@ -32,5 +32,9 @@ class Application < Rails::Application
     config.api_only = true
 
     config.time_zone = 'UTC'
+
+    config.i18n.available_locales = [:de, :en, :fr, :it, :gsw, :rm]
+    config.i18n.default_locale = :en
+    config.middleware.use Rack::Locale
   end
 end

test/controllers/application_controller_test.rb

@@ -0,0 +1,35 @@
+require 'test_helper'
+
+class LanguageTestController < ApplicationController
+  def authorize!; end
+
+  def index
+    render plain: I18n.locale
+  end
+end
+
+class ApplicationControllerTest < ActionDispatch::IntegrationTest
+  def setup
+    Rails.application.routes.draw do
+      get 'test' => 'language_test#index'
+    end
+  end
+
+  test 'reading the accept language header' do
+    get test_path, params: {}, env: { 'HTTP_ACCEPT_LANGUAGE' => 'da, loltheheck' }
+    assert_equal 'en', response.body
+
+    get test_path, params: {}, env: { 'HTTP_ACCEPT_LANGUAGE' => 'rm, de-CH' }
+    assert_equal 'rm', response.body
+
+    get test_path, params: {}, env: { 'HTTP_ACCEPT_LANGUAGE' => 'de-CH, en-US, fr' }
+    assert_equal 'fr', response.body
+
+    get test_path, params: {}, env: { 'HTTP_ACCEPT_LANGUAGE' => 'gsw' }
+    assert_equal 'gsw', response.body
+  end
+
+  def teardown
+    Rails.application.routes_reloader.reload!
+  end
+end