Welcome to the Auth0 Community!
Reading through your use-case, I would say that you should use the ui_locales variable in order to determine the user’s language and change the Welcome Email according to is. I highly recommend reading through our documentation on Universal Login Internationalization in order to learn more; you can pass the language code from ui_locales when calling the Auth0 SDK to log the user in.
Based on the language of your users, you will also need to enable them in your Tenant’s Settings page → Supported Languages to ensure it’s configured fully.
Allow me to share more resources that can help with this matter:
- Internationalization of login screen issue
- Customize Welcome Email Flow
- Localize email verification message
Hope this helped!
Gerald