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.
No it is not rude to speak another language in public in a private conversation. And where did you ever learn that whispering in public was rude? I’ll ask again why would the private conversations of complete strangers who are not acting in any suspicious manner be your business making it necessary for you to hear and understand it?
How do you feel about thought crimes?