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"Stargate SG-1" (1997) More at IMDbPro »

Anderson left the show after Season 8 wrapped to spend more time with his daughter, and O'Neill left the SGC after he was promoted the rank of Major General, and (presumably) to head "Homeworld Security" department.

In the original Stargate movie, the inhabitants of Abydos, the planet the inital Stargate team travels to, speak a version of ancient Egyptian which Daniel Jackson was able to translate. In a two hour movie with a three act structure, this makes the story more interesting and time can be dealt relating to the conflicts with it. In a 40-45 minute weekly show however, not enough time exists within each episode for Daniel Jackson or someone else to translate every language of every single planet that SG-1 visits (though one can reasonably argue that most planets in the Milky Way have sizable Goa'uld-speaking populations). Thus, for the sake of storytelling, the writers decided it would be best to ignore this issue completely. For the sake of the entertainment, it is best to ignore this issue and enjoy the stories that exist.

The reason given in the show is that all of the planets where they speak English were colonized with people from Earth by the Ancients, the highly evolved race that built the Stargate. Which is nonsense: At the time the Ancients came on Earth (let's say 5000 BC), the Indo-European tribes were still roaming around the Caspian Sea, and they were nowhere near speaking that mixture of old German, French, and Celt we today call "modern English". By the way, those aliens don't just speak English, they almost invariably speak American-English, and with American or Canadian accents to boot!

It should also be noted that there are a few episodes where there is a language difference and communication does not come easily. Rather than use the show time to translate and learn languages in most episodes, they usually skip this lengthy process to keep the show more interesting to watch.

taken from http://www.savedanieljackson.com/history/originalspirit/cine.shtml

Ive been playing the same character for a long time, says Shanks. I felt that the character was a regular and should be utilized like that. I thought that would be appropriate, but I felt that wasnt going to be the case. I was going to become a useless actor on that show. I felt that I should move on and do something else as an actor, so I decided to leave. But, adds Shanks carefully, our disagreement about the course of the show is not related to the wonderful years I have worked with these people. These people are great.

After doing 2 guest appearances during season 6 "Abyss" and "The Changeling", he realised that he really liked working there and missed it, and was brought back in the last episode of season 6 "Full Circle"