I recently acquired the full ten seasons of Stargate SG-1 on DVD from a co-worker back in October. The full series was being released on one boxed set, and he was looking to get it, thus freeing up much space in his home. I jumped at the chance - Stargate SG-1 has been one of my favorite science fiction shows for a number of years, combining science fiction, outer space exploration and human cultures into one show. There's only one major gripe that I have with everything in the show: Everyone speaks English.
On the hundreds of planets that SG-1 visits, no matter how remote or whatever culture on earth they represent, the team easily identifies themselves in English, and quite easily communicates with the locals. Given the huge variety of languages here on Earth, this is a really odd coincidence that's never quite been explained within the show.
Interestingly, most of the written languages in the show are different - most of which have derivatives of English, Latin, or a number of other earth based languages - one of th show's main characters, Daniel Jackson, is familiar with most of them, and typically spends just a couple minutes with any given thing before knowing what it says.
Visiting other worlds and alien cultures has been a longtime staple of Science Fiction television, and there are numerous means in which shows have surpassed the language barrier.
My favorite method is the one in Farscape - small cultures of microbes injected into someone that allows the brain to interpret other languages is used as a means of intergalactic communication. Prior to his own injection, and at various points in the show, we see John Crichton seeing the various languages in action, or other civilians on Earth being unable to understand aliens as they come into contact with them. Foreign languages are clearly acknowledged and utilized when necessary. (Non-humanoid aliens are a specialty as well in Farscape). A method similar to this was a creation of the late Douglas Adams, the Babel fish, which, when inserted into one's ear, could translate any language into whatever the host language was.
Another method has consistently been a translator, such as in various points with Babylon 5, where numerous cultures co-existed alongside one another, or in other cases such as C-3P0 from the Star Wars movies, a droid who's programming and knowledge encompasses millions of known languages. It would make sense that this sort of thing would result in specialized robots, given their ability to be programmed with numerous languages without the problems of training time.
Still other methods exist - one being to have characters know a number of languages - the short lived Firefly presented a world where numerous Earth cultures came together into a blend in space, with English and Chinese being the dominate cultures and languages. In this instance, Chinese was largely used for profanity, but for other cultural nuances as well. This worked exceptionally well for the show, which was one of the first contemporary science fiction shows, presenting a fairly realistic view of cultures in space. What's even better is that this all existed within one race - humanity.
Other shows, such as Stargate have largely eliminated the use of foreign languages. (Stargate does utilize other languages, or will have machines translate, but not consistently). One show that does easily get away with this is Battlestar Galactica, which likewise also utilizes a human-only species makeup, and which also presented a fairly unified inter-solar culture that would logically have had one dominate language. Still, localized languages would most likely have existed.
The lack of diversity in science fiction is most likely due to the sheer complexity of language and implications that has for both practical purposes and for the sanity of the actors. Logically, a show such as Stargate SG-1 shouldn't have gone beyond more than a season with just one planet and I imagine that watching the characters struggle to translate a language over an entire season, while academically interesting, just doesn't bring the entertainment science fiction fans are looking for. After all, there are people to shoot and things to find.