Is there a way to add or tag the Welsh language in Adobe Acrobat?

The Welsh language may not be available as an interface language, or may not be supported by a dictionary, but if you have a screen reader reading Welsh, it should also be able to read Welsh in an Acrobat file. Indeed, a language tag is not dependent on the interface support, but only dependent on the existence of an ISO code for that language. The code exists.

https://en.wikipedia.org/wiki/Welsh_language