Calamares, the Linux system installer for boutique distro’s, is translated into 50 or so languages. It’s not a KDE project, but uses a bunch of KDE technology like the KDE Frameworks and KPMCore. It doesn’t use the KDE translation infrastructure, either, but Transifex.
There’s two languages for Calamares that don’t have any translators — they were requested, and then added to the system, but there is noone to do the work. Those are Macedonian and Uzbek. That said, there are also translations to Lao, Farsi, Gujrati, Urdu and Swiss French (is that even a thing) that have not seen any actual translation work done.
If you’re interested in any of those languages, the Calamares translators guide can get you started.
PS.: boutique distro for me means anything outside the “big five”, it doesn’t say anything about the size or importance of the distro itself.