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/helpers/locales_helper_spec.rb

@@ -0,0 +1,32 @@
+require 'rails_helper'
+
+describe LocalesHelper do
+
+  context "Language names" do
+
+    let!(:default_enforce) { I18n.enforce_available_locales }
+
+    before do
+      I18n.enforce_available_locales = false
+    end
+
+    after do
+      I18n.backend.reload!
+      I18n.enforce_available_locales = default_enforce
+    end
+
+    it "returns the language name in i18n.language.name translation" do
+      keys = { language: {
+                 name: "World Language" }}
+
+      I18n.backend.store_translations(:wl, { i18n: keys })
+
+      expect(name_for_locale(:wl)).to eq("World Language")
+    end
+
+    it "retuns the locale key if i18n.language.name translation is not found" do
+      expect(name_for_locale(:wl)).to eq("wl")
+    end
+
+  end
+end