Generally, when a vampire compels someone, their eye pupil becomes large & victim listens vampire to follow exactly what he/she said. What will happen if victim is unable to understand language of vampire?
I have always assumed that a vampire's specific words are not a part of his power to charm, that it is rather either his physical presence (pheromones, a magical aura, or some such) or his voice itself (a form of magical hypnosis, perhaps also based on his incredible presence). If this is true, then the victim could be charmed/hypnotized without understanding any specific thing the vampire said, but would only take action to the extent that it could understand the vampire. Specifically, I'm suggesting that gestures and such could be employed.
I think where this gets interesting, is when you think about how this plays out in a book vs a movie. You could write a scene like this and not make it sound stupid, but visually, you run the risk of making the vampire seem comical, as any gesture more complicated than the "come here" motion runs the risk of serious misinterpretation. (When you consider the fact that classic vampiric charming seems to work by putting the victim into a state of stupefaction or a sluggish sort of dream state and not at their brightest, it makes the problem of communication more problematic.) I think an author who wanted to gloss over this problem would introduce some form of direct mind control.
Anyway, sorry for the ramble. It's an interesting question.