We use i18n.languages in the app-shell to see which languages are available
but this only contains a list of languages it will search in (means, currently
set language, default one and all fallbacks).

We can just set all languages we know as last fallback, so i18n.languages
always contains all languages.

A bit hacky...

Create a new instance with createInstance() and call setOverrides()
with override data to override translations.

See the tests for some examples.

Uses a separate namespace that gets used over the default one, so we can override,
but also remove the overrides again when needed.

This has the problem that all clones have the same store, so this overrides
the translations for all clones and not just the one providing the overrides.