Also, I'm not really sure the 100's of dollars each month assessment would be true. (That pricing is in characters, not words, or even lines.)
If the server requested the translation then every bit of text on every single channel and PM's would have to be translated into every other available language (or there would need to be a system to be aware of what languages were being spoken in which rooms) and then presented based on language preference of each client. One would get very expensive very quickly, the other would require a good bit of coding.
If the client did the translation then it would be on-demand and probably make more sense, but it would also multiply the number of requests for every foreign speaker in any given room or channel.
1,000,000 characters might seem like a lot, but it's really not. I'd be willing to bet that 3dx is generating 1,000,000 characters every few minutes. Not all of that would need to be translated, of course.
But, just consider how much translation the client would have to do for all the text scrolling through World Chat every second that it is online...that's going to rack up really fast.