Add translation status to language selection screen

_Transferred from [elementary/greeter#743](https://github.com/elementary/greeter/issues/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!

Inizia dalla schermata di selezione della lingua e segui il caricamento delle localizzazioni supportate. Determina come archiviare e aggiornare le percentuali di traduzione una volta per ogni release di elementary OS, quindi verifica il comportamento degli avvisi per le localizzazioni al di sotto del 90% e del 70%. L'indicazione proposta nell'elenco delle localizzazioni rimane una questione di design aperta.

Scritto dal modello di indicizzazione a partire dal testo della issue.