Improve i18n chooser (#1804)

* Add locale spec with failing locale plus region check

* Use a more accurate locale when supplied by browser headers

Previously we were using a matching option which would use the first locale
available which matched the locale portion, even if a region was specified.

This changes to first try to find an exact match, and then fall back to the
region, and then fall back to the  default.

* Clean up default_locale method

app/controllers/concerns/localized.rb

@@ -27,7 +27,11 @@ module Localized
 
   def default_locale
     ENV.fetch('DEFAULT_LOCALE') {
-      http_accept_language.compatible_language_from(I18n.available_locales) || I18n.default_locale
+      user_supplied_locale || I18n.default_locale
     }
   end
+
+  def user_supplied_locale
+    http_accept_language.language_region_compatible_from(I18n.available_locales)
+  end
 end

spec/requests/localization_spec.rb

+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe 'Localization' do
+  it 'uses a specific region when provided' do
+    headers = { 'Accept-Language' => 'zh-HK' }
+
+    get "/about", headers: headers
+    expect(response.body).to include(
+      I18n.t('about.about_mastodon', locale: 'zh-HK')
+    )
+  end
+
+  it 'falls back to a locale when region missing' do
+    headers = { 'Accept-Language' => 'es-FAKE' }
+
+    get "/about", headers: headers
+    expect(response.body).to include(
+      I18n.t('about.about_mastodon', locale: 'es')
+    )
+  end
+  it 'falls back to english when locale is missing' do
+    headers = { 'Accept-Language' => '12-FAKE' }
+
+    get "/about", headers: headers
+    expect(response.body).to include(
+      I18n.t('about.about_mastodon', locale: 'en')
+    )
+  end
+end