it is rather rude when English is being spoken, and rapidly switches to Welsh when a non-welsh person enters. The language barrier is a two way problem and there has to be tolerance on BOTH sides!
I've never known that happen, though, and I spend more time travelling around North Wales than practically anyone on this forum? As I said before, why would people who speak Welsh all day long suddenly decide to speak English? It's as ridiculous as the idea that you would walk into a shop in France and they would all be speaking English, then switch into French! 
When we talk about tolerance, it should be remembered it is the Welsh people that speak two languages and the English that speak only one. Who is therefore making more effort to be tolerant? 
The Welsh (or some of them) speak both languages because they have been brought up learning both from childhood.
My schooling concentrated on teaching the English language until the age of 11 then a foreign language was introduced for the last five years of Secondary Education. Had my teachers had foresight they would maybe have taught me Welsh instead of German!
In the mean time since leaving school, I have learnt some French, Portuguese and Japanese. My wife also has French and Portuguese. Consequently we are not ignorant of other countries or their individual languages and are tolerant of all of them, including Welsh.
Your comment about the French borders on the ridiculous as France is not a bi-lingual country and the French would be unlikely to be conversing in English!
Some years ago I came down from Scotland to buy some equipment that had been in use on the construction of Dinorwig Power Station. I had my engineer with me, who was a Welshman, but who had developed a slight Scottish accent. We went out for a pub meal and in the bar as we walked in English was being spoken, that is until after we had started our conversation, when it immediately switched to Welsh. The group spent the next half hour denigrating the British, or English. How do I know? - My engineer spoke fluent Welsh - and as we left he gave a mouthful of advice to the group and even managed to elicit an apology from them on my behalf. This is not the only experience of such bad manners that I, and no doubt others, have experienced.
I presume you are a Welsh speaker and such an action would probably go unnoticed.