To answer my own question - yes, you have to completely uninstall the previous BIMserver installation if you want to change the language code. I don't know why, but that's how it is.
And yes, uninstalling COMPLETELY NUKES ALL YOUR PROJECTS AND DATA (sorry for caps, bu I cannot stress this enough). The installer will show a warning, but the path in the warning is THE DEFAULT PATH for project databases, NOT necessarily the one it will nuke. Uninstalling completely erased all our projects from a dedicated SSD drive. Luckily, we had backups.