Limit language selection to installed languages

We currently use the predefined list of languages/codes from Drupal Core to populate the list for choosing which language the editor interface should appear in.
Since Core allows any language to be installed and used, even if not in that list, we should include those languages.
So, it would make sense to populate the language selection based on the table of installed languages in the database instead.

Doing that would also bring us one step closer to fixing #362318: Limit wysiwyg language selection to available languages.