Issue #2224887 by alexpott, Berdir, Gábor Hojtsy, Jalandhar: Language configuration overrides should have their own storage.

Issue #2219499 by Berdir, alexpott, Gábor Hojtsy, vijaycs85, swentel: Generalize language config override responsibility.

Issue #2215503 by Gábor Hojtsy, Wim Leers: Use services instead of event subscribers for config module overrides.