Miaghstir: I wonder why it's a choice at install time rather than a settings application run afterwards though. Sure, the installer is probably an easier place for most, but installing all languages and allowing the user to switch later would make it possible to switch without having to reinstall (but maybe I'm just weird in that way).
No, your're not weird. It would be nice to have the option to install multiple languages and have the option to switch ingame. I prefer to play story-driven games in my native language (easier to follow), but also enjoy to listen sometimes to the original language. The best solution for me is a mixed install, text in native and voice in original language (mostly english).
The option that you don't need to install all languages is of course also nice, less disc space is needed and a faster installation.
My guess: They used the structure of the CD release. Talking about Syberia hard discs were at the original release time much smaller, the option to install all languages was just an overkill.