You can change the language (user -> User settings -> Language). About them, I assume you have to fork Postmill (which is what this forum is using), copy the *.en.yml files while also changing the ISO 3166 code (e.g. to *.pl.yml), translating those new files, then making a pull request to the Postmill repository you just forked. By the way, I checked how many lines of messages are there, and it's 580.