[FIX] base: single lang behaviour

If a db is in single lang (en_US or not) but still has translations, the
translation should be updated in addition to updating the referenced record
Add tests to formalise the expected behaviour:

If one language only (en_US or not), when writing on a translatable field:
- record field should updated
- value of potential existing en_US translation should be updated
- src of potential existing translations should be updated
- no new translation should be created

If en_US and fr_FR, when writing on a translatable field in en_US:
- record field should updated
- value of potential existing en_US translation should be updated
- src of potential existing translations should be updated
- new en_US translation should be created if was not present

If en_US and fr_FR, when writing on a translatable field in fr_FR:
- record field should not be updated
- value of potential existing fr_FR translation should be updated
- src of potential existing translations should not be updated
- new fr_FR translation should be created if was not present

Adapt test_new_api test
get_installed is ormcached, just putting active = True is not enough