Fellow Americans,
I came across a 2021 movie called “The Feast” on Hulu (I have a basic subscription) and its all in Welsh!
This is a horror movie and is not a family movie and not really safe for work.
I will add more movies to the list if I find them.
Apologies if this was posted elsewhere.
Diolch!
Hello Tim. Here’s another suggestion for you. I enjoyed the film Patagonia, which came out about 10-15 years ago. The dialogue is in Spanish and Welsh and the locations are Argentina and Wales. Two stories for you to follow. I assume that you can find it out on the web somewhere. 
Patagonia came out in 2009 and is streaming on Kanopy in the US. Many public libraries will allow one to watch Kanopy for free with a library card.
Given that lots of American people were so surprised to hear Matthew Rhys speaking English in his natural Welsh accent, it’s going to blow their minds to hear him speaking Welsh with a south American accent! 
“The Feast” is a horror flick, but let me be upfront; it’s not something you’d want to watch with your family or during your coffee break at work. This one is strictly for those late-night, lights-off, heart-pounding movie sessions.