No problem, it's a pretty simple process. ^^
Click Start > Control Panel > Regional and Language options.
Administrative > Change system locale > Japanese (Japan) > Restart computer.
And that should be it! A sign that you're successful is, when most programs run, there is an (EN) symbol somewhere on the screen, and when C:/Users looks like C¥Users instead (or similar, that's just an example). Again this doesn't affect pretty much any programs you use, it'll just allow Togainu no Chi (and other Japanese language games) to work properly.
So after that just uninstall the game and reinstall it and the English patch. Then it should work fine!