Add API to get fallback locale from a key and 'global'

[yaustar]

At the moment, it isn't possible to get the fallback locale for a key in the case that it isn't localised in the current locale.

This would be needed to be able to do things like localise numbers correctly. Eg The game is using 'ar-EG' but the key is falling back to 'en-US'. In which case, we should using English numerals (0-9) as the text would be in English and potentially using a font that won't have Arabic numerals.

'global' is in the case where just the number is shown and therefore we should be able to check if a localisation exists at all for that locale.

[yaustar]

This has been implemented in #3245 (review)