When all your characters speak the same languages in a scene, you can use the most accurate translation of that language's idiom to convey some nuance. For example, the French say 'Cherchez la femme,' (Look for the woman) to suggest many things from the bad behavior in men to suggesting intrigue. There isn't an equivalent expression in English. But, you wouldn't want to use the French expression nor its literal English translation since it wouldn't read like a competent use of language. So, you'd want to use the ideas beneath the idioms to convey subtle biases and culture touchstones.
If the characters are from multiple cultures, speaking in a common tongue, then you can be more literal in the translations, since this would show the viewpoint character translating the idea 'Too many cooks spoil the soup.' Into their own culture.