When everyone speaks the same language, these side conversations are just a minor distraction. But in group discussions requiring an interpreter, people will feel left out and uncomfortable because they don't understand what the side conversations are about.
When side conversations occur in an interpreted group discussion, you should interrupt, saying, for example, "Excuse me! The interpreter reminds you that if we have side conversations that are not interpreted then some people will be left out of the discussion." Then you should interpret the side conversation or ask the people engaged in the side conversation to repeat what they said so that you can interpret for the rest of the group.