Make LocalisationCache a service

Make LocalisationCache a service

This removes Language::$dataCache without deprecation, because 1) I
don't know of a way to properly simulate it in the new paradigm, and 2)
I found no direct access to the member outside of the Language and
LanguageTest classes.

An earlier version of this patch (e4468a1d6b6) had to be reverted
because of a massive slowdown on test runs. Based on some local testing,
this should fix the problem. Running all tests in languages is slowed
down by only around 20% instead of a factor of five, and memory usage is
actually reduced greatly (~350 MB -> ~200 MB). The slowdown is still not
great, but I assume it's par for the course for converting things to
services and is acceptable. If not, I can try to optimize further.

Bug: T231220
Bug: T231198
Bug: T231200
Bug: T201405
Change-Id: Ieadbd820379a006d8ad2d2e4a1e96241e172ec5a
(cherry picked from commit 043d88f680cf66c90e2bdf423187ff8b994b1d02)