I believe you don't need AppLocale, but you need to set all your regional settings to Japan/Japanese. (And I've read somewhere that you can revert to your original settings after the RTP has been installed, and the game will work, but don't quote me on that.)
Since I study Japanese, I need to use some apps that were designed to run only on Japanese Windows versions, so I always have everything set to Japanese. The only visual difference is that some applications (most notably the command prompt) use different fonts than on US regional settings. The system language is still English. And I can play YN and all of the fangames absolutely fine. So you, and everyone else struggling with AppLocale, should try this.