[quote Heartofglass12345]@Zancah it's hilarious!
People wouldn't go to Spain and complain that people were speaking Spanish would they lol
South Wales isn't even predominantly welsh, I can only speak a bit of welsh.
How does your mum know they were speaking English before she walked in? Could she hear them?
It's a welsh conspiracy 
[/quote]
Look, I understand your stance on this, but funnily enough, English speakers can generally understand English when it's spoken, even as background noise. It has, like all languages, pace, cadence, particular tics, volume, emphasis.
This is why you get comedians who can appear to be speaking a particular language whilst actually speaking nonsense, made up words.
I can detect, at work when a couple of my NHS colleagues are no longer quietly conferring in English but in a shared Indian language. I wouldn't have been able to follow their chat in the preceding English, but I knew they were now not speaking English.
Disclaimer: I DON'T CARE.
But don't go assuming an English person doesn't spot 'the change' within a couple of moments if the language switches from English to Welsh, as they enter a shop of pub.