perf: prevent setting all languages for all apps on initial load

Description

Changes the way how app languages get set in gettext so only the current language is being considered initially. Other languages will be set dynamically when changing the current language.

Also refactors the language setup process a bit to make it simpler.

announceGettext sets up gettext in our Vue app, without any translations yet though. announceTranslations gets called later during the bootstrap process and provides all translations to gettext: the ones for core, additional ones and the ones for the apps for the current language. That way we have all that stuff in one place.

Related Issue

refs https://github.com/owncloud/web/issues/11035

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[ ] Technical debt
[ ] Tests
[ ] Documentation
[ ] Maintenance (e.g. dependency updates or tooling)

owncloud / web