Add translation status to language selection screen

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Transferred from elementary/greeter#743 (misclicked the repo, I am sorry)

Problem

Sometimes users complain about certain locales being incomplete, thinking of them as system-level flaws developers failed to handle (see Reddit). Furthermore, currently the only way for a user to check whether a locale is complete is to use it in Demonstration mode, which some might either only take a brief look at, or skip altogether.

Proposal

1. Store translation status percentage for each supported locale, update once per eOS release
2. When user selects a locale:
   • If translation status for this locale if 90+% finished, do nothing
   • Show light explanatory warning if the locale is <90% finished
   • If the locale is <70% finished, show more severe warning text
   • (the numbers are picked from Weblate colouring scheme and are subject to change)
3. Indicating translation status for locale selection list items too might be a good idea as well, however I can't think of a good design for this

The point of this feature is:

• Preventively warn users about largely incomplete/abandoned locales
• Make sure users understand unfinished system translation does not affect the system performance or stability, and might improve after updating the system
• Potentially engage users in helping translate elementary OS in the chosen language

Prior Art (Optional)

Coming soon!