Shadowfax is right, it's all a matter of file sizes. Basically, if voices are decided to become madatory they would be bundled with the chapter files (as I said earlier).
However, chapter files may be broken up into further smaller parts (otherwise the server load for Chapter 1 would be insane

). Does anyone remember how IE's Active Setup worked? There were maybe 20 cab files, but if you decided to stop at some point and start the installer again, you wouldn't start from cab 0, you would start from the last incompleted cab. The only thing I
didn't like about that was it didn't support resume capability.
So, let's say Chapter 1 is 400mb including voices/video/everything, the installer might look like:
- Chapter 1
-> Chapter 1 - Part 1/10
-> Chapter 1 - Part 2/10
-> Chapter 1 - Part 3/10
-> ...
Which means, broadband users could select Chapter 1 and it would download all 10 parts sequentially, but there is also the option (mainly for dial-up users) to ask for a specific part at a time, as each part would be 40mb. In either case, the user cannot play the game until all 10 parts of chapter 1 are downloaded and verified.
EDIT: And it was previously decided that you must play the game in order, i.e. starting from Chapter 1. So, downloading
only Chapter 6 will not trick the game into starting there.

After it's translated into other languages, we'll be able to listen to the voices, AND have another language for subtitles. Right? 
Correct. The game has three options: Voice & Subtitles, Voices Only, Subtitles Only. Translations only affect the subtitles, voices would still be in english.