Most of people aren't native english speakers, though I think we can still spot accidental spanish or out of box printing. 
Yes, but I think it would be more efficient to revise grammar errors, out-of-context phrases and typos along with bad formatted text if you are english native.
While I understand the idea of trying to spread different characters to dedicated testers to maximize coverage, I suspect that an open beta would be more efficient. Covering everything in a game with a lot of branches like this is hard anyway, and less organization/attention is needed if you just leave an open beta available and wait for reports. It worked pretty nicely for the RS1 project I think.
That could be a good choice when all miscellaneous text is translated, but I think it would chaotic right now. Just imagine hundred of emails reporting the same text with the same player, plus complaining about not all text completely translated into english.
Anyway, Einen emailed me to say that the remaining text could be finished in a short time, so maybe I could send a fully translated version to all of you who are interested on beta-testing.
There is no need to beat the game in a month, there is no deadline... I started that project 6 years ago, I don't think there would be any problem in waiting another half a year or so for a final release.