Oh, the Foreigners...
You do the best you can. Generally, if they're spectating a game, I'll ask them to keep silent. If a player in the match is talking with a spectator, I'll ask them to either stop, or speak in a language that the opponent can also understand.
If they ask why, I say that it's only fair on the opponent - they shouldn't have to worry about whether there is any outside assistance going on.
If someone reports people, I'll do the same as above. Although if the match is over then there isn't much I can do if I can't find them.
If they claim they're just talking about lunch then I'll take them at their word. There is little you can do unless someone witnesses something and tells you explicitly. And 9 times out of 10 they ARE talking about lunch.
In short, I generally ask people to remain silent, and hopefully that avoids any problems.