If you have ever come across a couple of smart-looking (a.k.a. geeky) dudes speaking in Klingon or Vulcan, then you already know the answer to that question. If people can learn languages that are not on this planet from watching movies, then people can certainly do the same thing with any language spoken on this world.
Maybe you have heard of people learning a foreign language by watching TV or movies. Some years ago there were reports that said actress Mila Kunis, who is originally from Ukraine, learned English by watching The Price is Right.
That didn't turn out to be true, but most of us believed it back then probably because it wasn't such a crazy idea to begin with. Many of us pick up lines or words from movies. Who hasn't quoted a famous line from a movie at one point or another?
Sometimes we even pick up foreign phrases or words from the shows or movies we've seen. I know many kids, real ones and ones who are “kids-at-heart,” who can effortlessly spout Japanese phrases or words. They got these words and phrases from watching their favourite anime.
Of course, they can't necessarily converse in Japanese when they have to, but it was easy for them to pick up these words or terms from the movie or show they watched. This doesn't really prove that you can learn a foreign language solely by watching movies. It only proves that watching foreign films can help any person become more familiar with a foreign language.
Many language learning centers are already using videos and digital media to help their students learn their target languages. Some of these language learning centers use educational videos to teach languages. Some learning centers use clips from movies to make learning more fun and interesting for their students.
With movies, having subtitles can help give language learners a clue on how words or phrases are pronounced or used in context. People who are trying to learn a language can become more familiar with their target language by watching a film in that language with English subtitles or if they watch it with the same language subtitles.
Having English subtitles help them recognise the words or phrases more. For more advanced learners, being able to read what the characters are saying helps them keep up with the conversation because, more often than not, the natural speakers of a language tend to speak faster and use a lot of slang.
People have different ways of learning a language. Some people learn more when they hear it while others prefer to see it or read it. Some people learn more through practicing and speaking the language regularly.
Whatever category you may fall under, you can't lose anything by using movies to help you learn a language. If you really want to learn a language, take a class or get a teacher. If you just want to confuse people by spouting lines in Klingon, then by all means catch a Star Trek movie and take notes.