Perhaps I am not explaining this properly. I’ll use an example.
If you are getting a haircut in a barber shop where the barbers are all Italian, it would be inconsiderate and rude for them to speak among themselves in the your presence in a language other than English, unless they had no ability to speak or understand English.
You are totally correct.
It is RUDE to a) speak another language or b) whisper in front of others in public space.
Both may mean the speakers are speaking about another person, or perhaps plotting something; anything nefarious.
It is not about “eavesdropping”, as the strawman argument goes. We hear stuff all the time that we simply cannot help hearing. Therefore, at least let it be something that is not directed at us personally or generally threatening. Using whispers and other languages implies the speaker is trying to hide something.
They have sharp scissors and razors. They can speak whatever language they like. Seriously if the level of conversation is becoming so loud that you are being talked over it would be polite for one of them to say “Please excuse us we’re just talking up the new Star Wars we don’t mean to be rude” for example. The apology more for disturbing you and not for speaking Italian.