“Just seen”

“Just saw” is accepted shorthand for “I/we just saw…” “I have seen…” is acceptable if you’re saying that you’ve watched a movie 27 times.

Even substituting in “I just have seen” in the OP doesn’t make grammatical sense to me.

The guy should probably worry more about his own English than other people’s Welsh.

Yup. Wish my Welsh grandfather could’ve lived to see this.