I had the experience recently during the forum challenge on
Titan of running into several players at the same tabel who were from a Portuguese forum. They were speaking Portuguese, and I could tell they were talking about the cards. I asked for English only, and one of them told me "This is a forum challenge, and we're a Portuguese forum, so we can speak Portuguese." They continued to do so. One of them took me out soon after. I can't say for a fact that there was collusion, but it was pretty shady.
I think if no one at the table objects, and it's clear that it's small talk, there's no harm done. But if any player asks for them to abide by the rules, and they refuse, I think that shows a bad intention.