Especially since given the way most DVD players function, if there’s a Japanese-with-English-subtitles option, you can turn it into a pure Japanese language option without too much fuss. If the DVD menu itself doesn’t have an option, you can still on almost any DVD player just turn off the subtitles from within the episode.
I prefer the R2 DVDs myself so I can get Japanese subtitles. Not that I need them that often, but sometimes I do. Native speakers of the language this doesn’t really apply to though I guess
Just got it today and already cleared. 
I agree, great fan disc, wish it was longer. The harem route was unnecessary though.
I’ve only played some of Sakura’s scenario, but I agree with DarkFusion that it is incredibly funny (but so was the main game for me). It’s rather rare for me to laugh out loud at anything, but this game does manage to get me going. I guess my humor is just that juvenile 
The translation quality isn’t even comparable to the original game, thankfully. If that’s the only thing keeping you from buying the fandisk, have no fear. It’s nothing like the original, horrible translation.
I think it can, yes. It has its own install folder and executables. Also, it doesn’t really spoil the other girls’ scenarios since they start with the main character having gone through the entire summer without finding a girl, i.e they happen later in the timeline than the regular scenarios do. You don’t even get to play the intro that shows how you end up on the Island and get to know the girls.
Personally I wouldn’t play the fandisk without playing the main game first, but not because it spoils the other girls’ scenarios, only because it dumps you in the middle and assumes you know what’s been going on.