Utaforum.net primarily caters to overseas members of the utau community (generally English speakers - be it as a first or second language). There's a [very] small number of Japanese language studiers and speakers here, but they're not very active compared to the rest of the community for whatever reason (work, school, personal reasons). Idk anyone who's identified as being ethnically Japanese here (and I don't understand the correlation that's kind of implied? Being part of an ethnic group doesn't always mean knowing or being a native speaker of the language of that group's country of origin - immigration makes a difference).
If you want to communicate with Japanese UTAU voice providers, your best bet is twitter, sites made by Japanese providers, (their email and other socials might be available) and niconicodouga. You'll have better odds at getting a response by communicating in Japanese, though.