Is there any way to know what languages are available on a given Itunes movie before I purchase it?
Yes. When you are looking at the move in the Store, look to the left side, and you will see the language in the line that starts with "Audio."
If the movie has closed captions and/or subtitles, that will be noted as well..
Here are a couple of examples.
And here is one that has subtitles:
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