Currently untested but updated to v26:
ar, cs, de, en-GB, es-AR, fr, ja, kn, nl, pl, pt-BR, ru, sl, tr, vi, zh-CN and zh-TW
Still in need of updates but salvaged:
So, I'm about half-way through doing this.
You can keep an eye on https://github.com/MoonchildProductions/pale-moon-localization -- and if you see any blatant mistakes, please suggest an improvement by making a pull request (editing a file on the github website will guide you through the process)