I've seen 3 ways to handle this problem:
- Dubbing the actor's lines
This only works if the actor has at least a small degree of proficiency
in the language of the movie. He or she has to be able to make
plausible movements of the lips, so that a native speaker can lip-sync
him or her. They did this in Arnold Schwarzeneggers debut movie,
Hercules in New York.
- Sub-titling the actor's lines
I've seen this done in a number of Scandinavian movies. One example
is in the Norwegian movie "Dykket" (the Dive), in which
Michael Kitchen played the role as Bricks. As
Scandinavia has such a small population, and thus a shortage of good
actors, they occasionally cast an actor from the English-speaking world
in their native-language films. The foreign actor invariably portrays
an Englishman/American/Australian/etc., who understands Norwegian,
Swedish or Danish perfectly, but speaks only English. This is not
as implausible as it may seem. Being able to understand a foreign
language comes much earlier than being able to produce sentences in it.
This applies both to the native Scandinavians, who have compulsory
English classes in primary school, and to the foreigner, who has
presumably been in an immersive environment since he or she immigrated.
- Not giving the actor any lines at all
By far the most ambitious solution. I've only seen this done once, in a
Swedish horror series. In one of the episodes, they cast Zelda
Rubinstein (of Poltergeist fame) as the protagonist's mother. The
son would say or ask something in Swedish, and the mother would
intone the reply: "Mm." "Mhm?"
"Mmmm!" I didn't think it worked very well. She must have had
a speech defect, or maybe she was retarded.