Hi, pascalegadbois.
Thank you for the question. From your question it seems like you are looking to change the language setting for a movie downloaded via Digital Copy. If this is the case, then the instructions below will walk you through how to adjust the language settings. Also, depending on the region the video was downloaded, not all languages are supported.
iTunes: Language settings for video
http://support.apple.com/kb/HT5562
Cheers,
Jason H.