Reply #72: In my area, the local traditional language of the Eno seems to have been

[struggle4progress]

.. displaced by a bunch of immigrants who came in and didn't adopt the lingo. The colonial populations and the natives they assimilated in the West and Southwest spoke Spanish before the English flood.

So your principle, that it's rude not to use the local tongue, seems to be a relatively recent discovery, having limited historical precedent.

I don't wish to dismiss the principle out of hand, because there certainly are times when it should be applied, as (perhaps) in Iraq nowadays, where American soldiers regularly kill civilians who fail to respond appropriately to commands shouted in English: that sort of thing really IS a major breach of etiquette, and in such cases providing a lecture on manners is the least we could do, appropriate as long as we are committed to doing the least we can do.