Use labels in controls to add and select languages

The absence of labels in these controls made them hard to use,
particularly for people who use screen readers.

Note we're removing the "Choose language" prompt, since we always
automatically choose a language and not choosing a language doesn't
really make sense. The only scenario where the prompt was used took
place when all languages had been removed but, in that case, the "Choose
language" prompt was misleading because there were no languages to
choose from.

app/components/shared/globalize_locales_component.html.erb

@@ -6,9 +6,9 @@
         <%= selected_languages_description %>
       </strong>
     </span>
+    <%= label_tag :select_language, t("shared.translations.current_language") %>
     <%= select_tag :select_language,
                    options_for_select_language,
-                   prompt: t("shared.translations.select_language_prompt"),
                    class: "js-select-language" %>
     <%= select_language_error %>
     <div class="margin-bottom">
@@ -24,9 +24,10 @@
   </div>
   <div class="small-6 large-3 column margin-top end">
     <% if manage_languages %>
+      <%= label_tag :add_language, t("shared.translations.add_language") %>
       <%= select_tag :add_language,
                      options_for_add_language,
-                     prompt: t("shared.translations.add_language"),
+                     prompt: "",
                      class: "js-add-language" %>
     <% end %>
   </div>