A few reminders:

Gaelic =/= Celtic

Old English wealh, wealas (>Welsh, Wales) wasn't the word for foreigner

English has more vowel-less syllables than Welsh

Welsh is not the oldest language in Europe by any metric

Multilingual people codeswitch everywhere

Also:

If you enter a room and find people speaking a Celtic language, there's a good chance they were speaking it before you came in.

It's not rude for them to continue.

If you feel left out, a good option is to start learning the language.