Well, I don't about Tiger and VoiceOver, but I am a native English speaker learning French and I have my Mac set to French. It's been really nice, and I think it helps beyond just computer vocabulary. I don't know if there is some grammar thing behind this, but all of the commands are given in the infinitive (Like Quit Safari or Hide Safari is said as Quitter Safari or Masquer Safari instead of the imperative, which is what I would expect (Quittez Safari or Masquez Safari). In my opinion, this makes it a lot easier to learn the verbs and compare them to the English equivalents. If it were in the imperative, one would have to unconjugate it, which could be difficult with irregular verbs.
I wouldn't know if they have a French voice in Tiger, but in 10.3 the computer speaks French text letter by letter, so it sounds like an American reading it from paper. As I was saying, I've learned much more than computer terms from it, and it definitely helps a lot.