Well, you’ll have to check and see if that module was included with the group of modules installed along with ProFTPd on your system. On my system, that was in /usr/lib/proftpd, but that location is going to depend on your distro and architecture.
Unfortunately, on the one system I checked here, I didn’t see a mod_lang – so it’s possible you’re going to have to manually compile that module.
If you compile the same ProFTPd version as you currently have installed – you could then just take the mod_lang module and install it along with the other modules you have.