I only speak English, and you're expected to do the same in a server that's PREDOMINANTLY English.
There's no rule stating that people can't speak languages other than English in NA/EU servers. People aren't expected to exclusively speak English on an English-speaking server; they're at most expected to be able to, but even still, they're allowed to speak whatever language they can and want.
Maybe the duo was having a conversation that they didn't intend you to be privy to? Maybe they just work best as a duo, and consequently only communicate amongst themselves? There's no telling what their reasoning could be, but the fact remains, they're allowed to converse in whatever language they want.
Choosing not to do so just stops all communication purposefully and is almost grieffing.
Would you rather they not talk at all? They don't have to do exactly what you want them to do, as long as they're playing to win and still following the rules.
Shit-talking in another language so they can't understand you is just one of a few things possible heinous acts to commit.
If you suspect they're insulting or flaming you in another language, you can report them for it.
