"Wyt ti'n boeth?"
Translation:Are you hot?
I’m guessing this only refers to actual heat? Or can poeth also mean sexually attractive like English “hot”?
The use of 'hot' for an expression of sexually attractiveness is not common to UK English and is unknown in Welsh. The much more common slang expression is the word 'fit' in UK English or 'ffit' in Welsh.
Perhaps not to the older generation, but "hot" is most definitely used to mean 'attractive, sexy' in Welsh/UK English amongst the younger generation and teenagers in particular.
"Pishyn" is another word; mae e'n bishyn - he's alright