@ortext For EX, your Windows environment HAS to be in Japanese every time you use it. If you own any Voiceroid 2 character, you can register on AHS's site to get access to an import file so that you can import your EX Kiritan into it, then you won't have the region problem anymore because the software isn't as strict about locale.
Anyway, for Windows, you have to go to:
Settings >
Time & Language >
Start setting each of the following one by one and testing if Voiceroid EX starts working:
Language: + Add a preferred language > Install the Japanese/日本語 language pack if it's not already (this is for Japanese IME)
Region: Country or region = Japan (Tells it your location is Japan) / Regional format = Japan (this makes your calendar in Japanese, I had a real problem with Voiceroid EX not working until I changed my calendar)
You might also have to go to Date & time and change the Time zone to Japan (UTC +9:00 for Osaka), but I can't remember.
Please let us know if this solves your problem or not.
Edit: Yeah, I know this is a thread for the locale patch, but I'm just explaining in general how the EX works, you'll have to ask
@Ulysses for help specifically with the patch.