Because if they spoke their native tongue you wouldn't understand them and it would add more to the cost of the production to put translations under them when they spoke.

Not all of them speak English, some are subtitled.

Because it is a convention to help move the story along without having to take the time to translate the dialog of the aliens. If every new group spoke their own language (as would most likely be the case in reality), then the characters would have to take the time to learn that new language and/or teach those people English. By skipping over that, they can more effectively move the story along in the limited time that they have to tell it. *************** Somnium, They don't usually make a big deal about it in Star Trek, but they do use a universal translator to allow them to be able to communicate with other species. However, there have been episodes where the UT's failure to work properly have created problems for the crew. With respect to the other Sci Fi programs, hey, it is fiction. You need to learn to suspend your disbelief to a certain extent. I don't mean that in a critical way. I have my own idiosyncrasies with respect to this. I don't got to disaster movies while they are in the theaters. I do this out of consideration for the audiences. I don't want disturb their enjoyment of these movies as I scoff at what passes for science in Hollywood. ;-) BTW, Somnium, what is the mountain you use for your avatar?

In Star Trek they had a device known as a "Universal Translator" Most members of The United Federation of Planets spoke a mutually agreed upon "Galactic Standard" language which sounds a lot like English. As for Stargate, it may be a side benefit of the gate/portal.

This was answered in the original and in voyager. beacuse of the diversity of beings all humans have a universal translator fitted so they hear the words in their own language. In voyager the U T is broken so they cannot understand the aliens and in the original series Chekov explains that he hears russian, whilst spock says he hears vulcan and of course kirk hears english. A great explanation but unfortunately the lip movements kinda spoilt it.

That's what I like about Farscape. It is explained from the start why everyone can understand each other...translator microbes are injected at birth. Now how come no one on the Star Trek franchise knows about translator microbes? LOL

Hmm! I guess dubbing is a blessing!

cause they know english