Because you're based in Japan, that's why you can only see the Japanese voices. To change the language of the voices, here are some methods that you can try.
In addition to changing your language preferences in your TikTok account, I suggest you change your phone's language settings as well as the region. Try using English (United States) or English (United Kingdom) and make sure to disable any location services, then restart your phone.
Alternatively, install a VPN to pretend that you're based in an English speaking country. If necessary, uninstall and reinstall the app. You may also need to remove your SIM card because TikTok may use it to identify your current region.
Hope that helps.