A major fact to remember in writing scripts is: Audience = Reader.
With that in mind itís clear - if you confuse the reader the script stands little chance to become a movie. Thus, whenever possible write in the language of the eventual reader.
In case that itís important that the characters speak in a foreign language, there are a number of options:
Option 1: Parenthetical
Use a wryly (parenthetical) to clarify that the actor will speak in another language: