You can also switch to a different font (this also helps if you ever publish in different languages like it happens in the EU) when you switch into the foreign speakers' perspective. The character shouldn't understand, yet you still have an omniscient narrator. If it's first or second person, from a character's perspective that doesn't understand the spoken language, writing speach bubbles in the foreign language and using captions to translate is a good move.
How you write foreign languages also depends from who's perspective are you telling the story.
