After completing a few hours of MangaGamer's stilted, bland, but otherwise fine localized release of ef - the first tale, I realized that I understood virtually all of the spoken dialogue, and decided it was worth pushing myself to read the unspoken segments, as I was starting to get really invested in the game, and playing in Japanese would be a great learning opportunity.
Unfortunately for me, just getting my hands on a copy of the Japanese version of the game was extremely difficult. Unless you use shopping service like Noppin, you cannot import one at all. And just give up on finding one new. Minori stopped printing copies ages ago. As always, I will not condone piracy on this blog. However, if you want to support the developers directly, finding a used copy of the Japanese release will not accomplish this any more than piracy would. If that is your goal, it would be better to either import Minori's newest title, 12 no Tsuki no Eve, or at least buy the MangaGamer release of ef, and then just acquire this version of the game by whatever means you can.
OK, so you've gotten your hands on a copy of ef - the first tale, and you're ready to install it. If you have never installed a piece of Japanese software before, you should begin by going to the control panel > Clock, Language and Region, clicking on "Region," and in the new menu that pops up, going to the Administrative tab. There, select "change system local" under "Current language for non-Unicode programs" and changing it to Japanese. You will need to restart your computer to complete the change.
Now that you have that out of the way, you go to install the game, but immediately get the following error message「ディスクの認証には管理者権限が必要です」which translates to "root privileges are required to authenticate this disk." This was fairly straightforward to figure out. As it turns out, Minori released a patch for Windows Vista/7/8 that addresses this particular problem during the installation. You used to be able to download that patch from their site, but they have since taken it down. I was able to find this patch, however, which allowed me to install the game with no issues on my Windows 10 machine. It even acts as a no-CD patch, which is good if you did purchase a used copy, as you'll want to keep that disc safe.
It should have been smooth sailing from this point, but unfortunately, the nastiest error had yet to raise it's ugly head. Upon trying to run the game, you will be greeted by the following error:「この環境では動作できません」which translates to "[the program] cannot start in this environment." After many hours scouring the internet, I came across this Chinese blog post explaining the laughably-simple fix: all you have to do to get the game to launch is change your computer's clock to the Japanese timezone (GMT +9)! That's it!
The Japanese version has English menu options
I hope this guide was helpful to at least one person, as I wouldn't wish all of the hair-pulling I did trying to get this to run on anyone. During my dive, I discovered a way to extract and inject assets into the game, so I may develop my own HD Remaster at some point, as it shouldn't be too difficult to upscale and clean up the art assets using Vapoursynth. In any event, for now, I'm just going to enjoy my Japanese porn game--satisfied with the justification that playing it is a "learning opportunity."