This is essentially a dubbing scene. The foreign film dub is done so often that it requires a note of its own. The players that are acting out the scene in the none existent language must be very clear in defining their environment and making solid physical offers for the translators and audience to work off of. It helps the players that are talking in gibberish to think of what they are saying in English. The standard translation gag is making a really long gibberish sentence and translating it to one English word, or vice versa.
Translators can also struggle for the correct English word, caught in gibberish idioms.
Translators can also refuse to translate things that are too crude in English, gibberish censoring. Another gag is where the translator rambles on and the gibberish players stop him and end the scene in chaos. Use a real language. Translate an opera.