I fear that there is no proper workaround for this at the moment when using the email template mechanisms in Auth0, since at the time when the email is sent out, the Rules engine hasn’t been passed yet (so no useful info in user_metadata) and also no access to context, request (parameters), redirect_uri, or anything alike that could help to identify the proper locale for the user.
The support for proper email localization is in the product backlog, so it’s definitely taking into consideration by the product team, but no info yet on when that would be available.