There are very few times it's OK to be on a speaker phone in public, IMO. One example would be if someone was giving you directions on how to do something and you needed your hands free to do it, such as change a tire, etc.
soooo rude!!! my landlords do that. both speak English very well but when they don't want me toI do think it is rude if you are in a restaurant or at work having a conversation with people and two of them switch to speaking in a language that the other person or people don't understand.