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?
I cannot believe you have no clue about ettiquette.
So you like when people are whispering around you and don’t get the weird feeling they might be laughing at you (esp. when they’re giving furtive looks your way). Of course, they could also shyly be commenting on your magnificent looks, but one can’t be sure.
Ever watch the Seinfeld episode with the Koreans and Elaine? Fictional example in point, very realistic.
There is also an old Civil War story about 2 prisoners on a train with a guard. They spoke Swedish or something and assumed no one else knew, so they tried to come up with plans to escape the train. Too bad the guard nearby knew what they were saying.
I can’t believe you never incidentally hear people talking and what they’re saying out in public. Give me a break. Do you righteously go around closing your ears and shouting “I can’t hear you, I can’t hear you!” when they’re in easy earshot?
This is why, just as an example. Don’t think it happens in real life?
https://www.youtube.com/watch?v=kn_id7JQH0o
https://www.youtube.com/watch?v=ycDfwns61Tk&t=14s
And here are some related items mostly whispering, but much on foreign speech as well.
https://www.thespruce.com/whispering-and-other-rude-communication-1216730
http://work.chron.com/workplace-etiquette-whispering-11842.html