I'm not a programmer, but I've been using mIRC for more than 10 years because I really find it the best IRC client for Windows.
Probably not the best solution, but to expose all strings in a file called en_EN.pot via gettext and then upload to Crowdin.com could be an idea?
This lightens the long and hard work of translation, also because, with respect, I don't think you know all the languages of the world. Also, many other people could help out.
In this way, translating the main en_EN.po file with simple software such as notepad ++ or even better with PoEdit or through the crowdin.com site would be a job for everyone. Many users use programs like "Resource Hacker" to translate mIRC, but it is certainly not the best way because they risk compromising the proper functioning of mIRC.