Why is a stranger’s conversation in any language any business of the surrounding people? We’re not writing of terrorist plots here. Is eavesdropping some sort of civil right that I am not aware of?
It is common courtesy. Does it offends you to consider the comfort of others?
It is simply accepted as common courtesy to speak the native language of the country you are in unless you simply never learned how. This is especially true when you are in the presence of Americans, in this case.
How do you know that?
And even if we dismiss terrorist activity - What if it appears to all intents and purposes that they are talking about you (pointing and giggling)? Shouldn't that be considered impolite?
Regards,
How do we know that?
We don't.