Why not use Nicotine+ instead? https://nicotine-plus.github.io/nicotine-plus/
If you really want soulseek qt then you can get it here http://web.archive.org/web/20210729024552/https://www.slsknet.org/news/node/1
Soulseek has not always supported UTF-8 character encoding, which is what is required to display Japanese (and other Kanji, and a bunch of other things). Soulseek NS definitely doesn't support it, so it makes sense to assume that older versions of Qt don't either.
I'd be interested to know your reasoning for installing a 32 bit OS.
I recommend using Nicotine+; it is frequently updated and there is a 32 bit installer for Windows available.
Seems like there are lots of stability and performance improvements: https://nicotine-plus.github.io/nicotine-plus/NEWS.html
Gotta give it a try; I found this last version unusable because of how often it froze.
I have no advice for the original QT client but, you can try using the alternative which is Nicotine+. It's an open source client that interfaces with SS so you'll get that same experience with better performance IMO.
Site is down for me aswell but I downloaded it with the Nicotine+ frontend and it works and looks great.
The .dmg installer won't work on macOS 10.14 or older due to being built on 10.15. Your options are to upgrade to macOS 10.15 or newer, or install Nicotine+ with Homebrew instead. https://nicotine-plus.github.io/nicotine-plus/#stable-high-sierra1013-and-mojave1014