John August has this advice:

• If it’s just a word here and there, and the meaning is obvious in context, don’t bother translating it. An example is when a ubiquitous foreign bad guy shouts at his men to do something. Since it doesn’t really matter what he’s saying, just use the foreign word if you know it. Sometimes, this type of dialogue doesn’t even make it on to the dialogue line, and gets summarized in the action like, “Moldona SHOUTS at his men to stop the angels.”

• If you think the dialogue would probably be subtitled in the movie, italicize it in the script.

• If characters are speaking in a foreign language for the duration of a scene or scenes, put a parenthical like “(in Russian; subtitled)” for the first speaking character, then just use italicized English for the rest of the scene or scenes.