Add translation shared partial

- Add methods to local helper helper to provide language names and translate content info into component format
- Add methods to ContentItem to provide available translations

Commit audit trail:
- app/helpers/locale_helper.rb alphagov/government-frontend@90f0dd8
  https://github.com/alphagov/government-frontend/blame/517e4d4619595806bc9a1fb904b2badb65332eaf/app/helpers/application_helper.rb
- app/views/shared/_translations.html.erb alphagov/government-frontend@887399c

app/helpers/locale_helper.rb

@@ -6,4 +6,30 @@ def lang_attribute(locale)
   def page_text_direction
     I18n.t("i18n.direction", locale: I18n.locale, default: "ltr")
   end
+
+  def native_language_name_for(locale)
+    I18n.t("language_names.#{locale}", locale:)
+  end
+
+  def translations_for_nav(translations)
+    translations.map do |translation|
+      {
+        locale: translation["locale"],
+        base_path: translation["base_path"],
+        text: native_language_name_for(translation["locale"]),
+      }.tap do |h|
+        h[:active] = true if h[:locale] == I18n.locale.to_s
+      end
+    end
+  end
+
+  def t_locale_fallback(key, options = {})
+    options[:locale] = I18n.locale
+    options[:fallback] = nil
+    translation = I18n.t(key, **options)
+
+    if translation.nil? || translation.downcase.include?("translation missing")
+      I18n.default_locale
+    end
+  end
 end

app/models/content_item.rb

@@ -43,6 +43,12 @@ def method_missing(method_name, *_args, &_block)
     end
   end
 
+  def available_translations
+    translations = content_store_response["links"]["available_translations"] || []
+
+    translations.sort_by { |t| t["locale"] == I18n.default_locale.to_s ? "" : t["locale"] }
+  end
+
 private
 
   def ordered_related_items(links)

app/views/shared/_translations.html.erb

@@ -0,0 +1,6 @@
+<% if @content_item.available_translations.any? %>
+  <div class="govuk-grid-column-one-third">
+    <%= render 'govuk_publishing_components/components/translation_nav',
+        translations: translations_for_nav(@content_item.available_translations) %>
+  </div>
+<% end %>