If "how accurate" means "how many straight-up mistakes are there", then there aren't very many of those. Typos and formatting issues affect maybe 1 in 20 text boxes. Contextual mistranslations only happen once or twice a chapter.
Have you ever looked at the script of a movie you really like? You know how the plain text on a plain white page can come across in a very different way than the real lines in the movie? The same thing happens in reverse. There are all kinds of things that look good enough when they're in a text file, but are a bit out-of-wack when they're playing out on the screen. Most of what I do during play-testing is just re-wording things to make them sound more natural and flow a bit better. That's kind of vague, I know, but if you want to know more of the nitty-gritty, that will have to wait for another day when I can dig up some examples.
In addition, there's the simple fact that most things benefit from editing and revision. Translation is not an exact science, that's for sure. You write something, you come back to it a week later and think you should change this, that, and the other thing. The best way to judge how well a translation is working for something like this is to see it happening in the game itself.
When I do the "proofreading" after I get a chapter to 100%, I'm really just scanning for typos, missed spots, inconsistencies, and anything else that's easy to fix.
