I was unaware of this website actually. It seems like one of the best method for translations I've heard of so far (even better than SVN in so
me ways as it requires no set up for each user). I'll have to think about using this for Diaspora.
My only complaint is that you started with tstrings.tbl. Strings.tbl is a better choice in many ways as it is less work but would work for FS2, TBP, WCS and every other mod or campaign based on the FS2 engine. Tstrings on the other hand only translates the main FS2 campaign.
Still that's rather minor if you manage to get both of them translated in the end.

Wouldn't it be simpler if you did a systematic mission-by-mission translation as opposed to asking for translating random sentences?
No it wouldn't. The second the next media VPs come out he'd have to patch all his missions or have them go out of date. Translating strings and tstrings is the only sensible way to do a translation.