According to this document Adobe Experience Manager Help | Supporting new locales for adaptive forms localization you need to follow the steps to add a custom language.
It works in AEM Forms 6.3 but in AEM Forms 6.4 the path of etc/clientlibs/fd/xfaforms is changed to the libs folder and the documentation isn't updated with version 6.4. Also overlay the libs doesn't seems to work.
My question, how we need to implement it into 6.4, what are the steps to add a custom language to a form.
Christofhe De Bock
It is known issue. Documentation team is working with Engineering team to fix this soon.
If this is really urgent/blocker for you, please create a support ticket.
Did you find a solution for this ? I'm trying to define a new locale, but it doesn't work.