Display language name or language key

There where two issues with the current implementation:

- There was a possible duplication between looking up the language name in key "locale" and in key "i18n.language.name"

- The "default" option was not being picked up, as the fallback always returned the default locale's translation, "English"

With this implementation there is only a single place to put the language name: i18n.language.name. I think this place is easier to find and understand for Crowdin translators than a "locale" key hidden in general.yml

If the translation is not found we display the language key, instead of English, which makes more sense to me too 😌

Solution based on recent comments[1] on a related I18n issue

[1] https://github.com/svenfuchs/i18n/issues/365#issuecomment-419263847

spec/features/localization_spec.rb

@@ -48,4 +48,30 @@ feature 'Localization' do
     expect(page).not_to have_content('Language')
     expect(page).not_to have_css('div.locale')
   end
+
+  context "Missing language names" do
+
+    let!(:default_enforce) { I18n.enforce_available_locales }
+    let!(:default_locales) { I18n.available_locales }
+
+    before do
+      I18n.enforce_available_locales = false
+      I18n.available_locales << :wl
+      I18n.locale = :wl
+    end
+
+    after do
+      I18n.enforce_available_locales = default_enforce
+      I18n.available_locales = default_locales
+    end
+
+    scenario 'Available locales without language translation display locale key' do
+      visit '/'
+
+      within('.locale-form .js-location-changer') do
+        expect(page).to have_content 'wl'
+      end
+    end
+
+  end
 end