Hi,
We’ve stumbled across the problem as well. TL;DR is that Auth0 does not offer first-lass support for this, nor are there any plans to support it in the near future. Most of the responses from Auth0 suggest leveraging liquid syntax conditionals in your templates based on the user’s locale. The suggestion for accessing a user’s locale is to save it inuser_metadata and reference it in your templates. This is also an issue because it can quickly get out of sync with Universal Login pages, as they do not account for user metadata locale, but from system/browser locale(How to detect language in the Email Template base on trigger page's language? - #4 by NeilShang).
Stumbling upon this Community Feature Request, there’s a custom tool for this. We ended up using the same approach.
Hope this helps!