refactor: provide use_fallback to get_cached_translated_field() too

Both methods share the same logic now, only return different results
refactor: replace internal "None" with MISSING sentinel object
Avoid query for fallback language if the cache knows it does not exist.

* Insert the MISSING value into the instance cache
  (since cache functions always return * None)
* Also set the __FALLBACK__ marker for the fallback language.