I completed a feature to enable multiple languages in the launcher, however the launcher is due for significant changes so I abandoned this work.
At the app level (eg demo app, email app), it will depend on the vendor to provide translations for their particular app.
Vaults are a background process, but it’s possible there will be interfaces in the future if the vault rpc feature is implemented, where it will depend on the developer of the interface to include multiple languages.
Hopefully this clarifies the current situation.
I would love to see i18n considered from the very start for apps, since the majority of users will probably be non-English speakers.
edit: Also have a look at the Translations thread for more info about this topic (more about the documentation than the software).