MVC: Illustrate translation coverage in language selection dropdown

As a first step 👣 toward #18923, we can make the translation levels hardcoded and manually update them on a cadence.

Proposal

• In the drop-down, show to the user the % of language translated.
• Allow for manual updates of the hardcoded translation levels.

Design

Add (x% translated) to each language choice. For example, instead of French - français, display French - français (88% translated).

Future iterations

• Technical follow-up: #329326
• Complete solution: #18923