This is consistent with the implementation used in the (now removed) webL10n-library, and by only using lowercase language-codes internally in the `L10n`-implementations we should avoid future issues e.g. when users manually set the `locale`-option (in the default viewer).
To prevent the *standalone* viewer-components from breaking, we need to ensure that the `NullL10n`-interface won't accidentally diverge from the actual `L10n`-implementations.
- For the generic viewer we use @fluent/dom and @fluent/bundle
- For the builtin pdf viewer in Firefox, we set a localization url
and then we rely on document.l10n which is a DOMLocalization object.
