Fix 0 pluralization for some localization strings (#17576)

Apparently, when the `zero` case is not defined, the translation string used
for `0` depends on the language. In French, `one` is used, causing some strings
with hardcoded “one” or `1` to misrepresent the actual data.

For instance, the dashboard would display « 1 utilisateur·rice en attente » for
both 0 and 1 pending users.