I’m pretty certain there’s a golden retriever in there…
Not a sentence I thought I’d ever say.
At least you’re just saying it, remember Old Scratch? He once admitted to performing analingus on a golden retriever.
So, you were where no one could see you and you heard them clearly speaking English, then when you walked into the room, everyone looked at you, then started speaking Welsh? And you talked to them and they acted like they couldn’t understand you.
Also, so? You are not entitled to hearing people’s private conversations. What they were talking about was Not Your Business.
This is part of a whole range of calumnies applied by the English to people from Wales:
“Taffy was a Welshman, Taffy was a thief” (A well-known nursery rhyme)
Various “jokes” about sheep-shagging.
“welsh on your debts”
The Welsh language includes many English words that have no translation, so a visitor to a pub in a Welsh-speaking area may well hear some English words mixed in.