Trilarion: That's interesting. I find it more naturally if all rights of a product are concentrated at one party. What does the translator want to do with the translation? After all it's specific to the product. Maybe sell it separately as a book? You also don't want to buy your car in parts from different companies.
Maybe it was just limited foresight in the old days. Publishers were divided into geographical regions and acting quite independently. They negotiated not the full rights because they didn't think enough. Like they thought a translation will only be needed in a certain geographical region instead of the whole world and GOG is selling worldwide every language. That might explain why Steam has more languages for some games than GOG.
hedwards: Yeah, I'm with Trilarion, what good would owning the rights to the translations be without the rights to the game? It's not like you can sell the translation separately from the game or that somebody can't just do a new translation if need be.
I'm not entirely sure how it works for games - a topic my lecturers presumably have very little, if any, experience with - but let's say you do the subtitles for a documentary film about elephants that's aired on a fictitious Channel One. The person you negotiate the contract with agrees to let you have the rights to the subtitles for whatever reason - you suggest a lower price, he makes a mistake, you give him a handjob or something. Sixteen months later Channel Two wants to air the very same documentary; they already have the rights to air it, but no subtitles. One of the Channel Two executives taped the documentary back when it was on Channel One (yeah, he's still got the Philips VCR he inherited from his father), watches it again and decides that the subtitles are spot on. Joy of joys, you hold the rights because you insisted on it even though you didn't know whether you'd actually profit from them. The Channel is pleased with your work but still tries to figure out whether they could get a similar-quality subtitling from a freelancer for less money. Then disaster strikes; a natural disaster, war or a riot causes Superbowl to be cancelled for the year, so the channel decides to fill the vacuum by airing the documentary. Naturally, this creates another vacuum further down the line, but they can deal with it later; for now, getting the subtitles quickly are a priority. From your point of view, you're getting paid for work you've already done; Channel Two, on the other hand, most likely doesn't care who or where the subtitles come from and how much time has been used on them, as long as it suits their needs.
An example from literature, then. Back when you were a poor student, a small publisher contacted you and asked you to do a French translation for a novel. You figure you could really use the money and agree to do the translation; you check the text and manage to convince the publisher that the pay is quite poor for the work they expect you to do, and that you could still do the translation for them if they agreed to let you have the rights. They say yes, get their translation and publish it. The novel flies in under the radar at first, but a literature journalist gives it a shot and praises it greatly. Word gets around, people read the novel and love it to bits. Hoping to capitalize on this, the publisher decides to republish the French version in e-book format to spread the joy. The original contract only gave them the rights to use the translation in printed format, so now they buy the translation from you again to use on e-books - it was your translation that the people loved, so of course that's the one to use. Besides, rebuying the rights costs the publishers about the same (possibly less) as having the entire thing translated all over again, so it's not like there are any saving to be had by retranslating it.
So yes, you can sell a translation separately. And yes, a game or a book could be retranslated by someone else, but I'm sure a publisher would rather pay a couple of thousand euros to a translator they already have experience with than risk waiting several months for a substandard translation by somebody else.