Improve Translation User Interface

Problem/Motivation

Translating email templates is now possible in the 3.0.x branch. However, the user interface is not great and could be improved. It's using the default Drupal core interface, which looks like this:

That's functional, but it's not as easy to use as the custom form we have under Edit Template.

Proposed resolution

I think the translations list page is fine, but ideally, when you click Add or Edit next to a translation there, it would open that translation in the custom form that we have under Edit Template. On save, you'd be redirected back to the listing page.

Also, we don't have the ability to preview a template in different languages. It would be nice to add a language switcher of some kind to the top of the Preview Template page. The language switcher would only be shown if there was more than one translation for the template.

Remaining tasks

Code the new feature.

User interface changes

New interface for doing translations.

API changes

None.

Data model changes

None.