Can I change the language the game client uses?

Solution 1:

Yes, you can change the language. And yes, it downloads the extra files when you do (after restarting the game client).

I changed it to Spanish (I had it at 100% in English) to try it for you and it's now updating.

I really wish it would've warned me because now I have to wait for it to reach playable before I can re launch it and change it back. I hope this answer prevents someone from doing it without knowing it.

Luckily, reverting back to English did not download anything additional.

If you want to change the language, you need to go to Account, and then select the desired language. This changes all text and spoken dialog in the game.