You definitely need to change the actual locale of your machine to Japanese/Japan (UTAU is a Japanese program; the coding and by extension most voicebanks that you download will be corrupted with gibberish/mojibake cause your machine can't actually read Japanese proper) and reinstall UTAU all together using a new download. UTAU comes in (mostly) English, so language packs are unnecessary.
- Latest version of UTAU (mostly in English):
http://utau2008.xrea.jp/utau0418e-inst.zip
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http://www.isunshare.com/windows-10/change-system-locale-in-windows-10.html - how to change locale in Windows (instructions are more or less universal between 7, 8.x, and 10)
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@Adlez27 pretty much explains resamplers well, so no need to reiterate on that front. Only change I would make to that is that for Tool 2, you don't have to navigate your file system. If your resampler executables (.exe) are saved to the same directory as UTAU, then you can simply type their names in with the extension .exe - not case sensitive.
If you're concerned about what locale will do your machine: certain programs will install with a Japanese interface if there isn't an option to control it during installation (for ex. Skype lets you pick your language). Most programs can be changed to an English interface via preference settings (simply google for programs that you aren't familiar with as far as changing preferences). The only time I couldn't do this was with the DAW Studio One. If you have programs that you absolutely need to run in English where you can't change the language through settings (like the aforementioned Studio One), then you might want to install those first and make UTAU the final installation in your PC setup (install English programs before changing your locale; then change your locale to install and run UTAU). You won't have to learn how to read Japanese with Kanji or anything like that. Your machine will still have a mostly English UI, but will be capable of running programs originally made in Japan.