On the PS3, it was easy.
There are a couple of relevant menu options where you can set things like the audio language, the subtitle language, and the menu language.
If you've got the Tomb Raider game that's sold in the US, I believe the fully-translated options were:
I finally decided to try installing the PC copy of Tomb Raider I bought during a sale so I could futz with the different language options.
I had activated the game on Steam, so I can't speak to other ways to launch the game, but to me, it wasn't very obvious how to change things.
If you just launch the game as-is, you do get the option to change the game menu language to several different options.
But if you're wanting to get a fully localized copy, you have to right click on the game before launching it. Then in the properties menu, there's a language tab.
I don't know if the other steam games are like this, but here's something that I really think sucks: everytime you want to switch languages, you have to redownload a language. As far as I can tell, it doesn't keep what you've downloaded. And at one gig a pop, it's a non-trivial wait if you want to futz about in a different language.
I must say, I can kind of understand it. Probably 99% of the people out there are going to set the language once and then call it a day. In other words, they won't be switching back and/or switching around. So, for them, keeping the other language packs installed would be a waste.
Like I said, I don't know if all steam games are like this, but I would definitely like being given the option of which languages to keep/remove.
Like, I might want to dip my toes into Russian or Arabic or something, but I'm probably not going to be very motivated to do it if there's a 15 minute wait before I can do it. And then another 15 minute wait before I can play in something else.
Frankly though, I'm thrilled this sort of thing exists at all. Like for instance, as far as I know, I'll never have the chance to play Uncharted in German because it's not included on the North American PS3 disc. But in a similar situation with a Steam game, it appears to be that all available languages exist for anyone who wants to download them. With the exception of Japanese, because I swear I heard there's a Japanese version of Tomb Raider and it doesn't show up as an option for me.
Not that I'd know what to do with a Japanese version at this point though. Arabic was terrifying enough since I basically had to have the menu locations memorized to be able to do anything (which of these scribbles means "new game"?).