Request type forms allow setting their translation under language support and are automatically translated to the customer language - the same is not true for forms which only allow setting one language.

It would be good to have the option to have language support for Forms for multi language customer portals.

Workaround 

This article outlines how you can effectively utilize Jira's native functionality to showcase form content in a user's preferred language.
> Forms : Display the Jira Form in users preferred language