Using the language loaded from cleartext doesn't really set the language

The keymap is set correctly, but the language isn't. This has been reported by wb:b708fe295ad260e57a1b9fa0d9e39812 and wb:5b946aa240eaee0bfc240a6fe822139e

The workaround is to manually set the language.

In a system where everything works, /var/lib/gdm3/settings/transient/ has the same contents than in a system which exposes the bug.