To add a new man page just write it in .pod format and add it to the list at the top of CMakeLists.txt. Then rerun cmake. Make sure to add the $Date$ and $Release$ lines in the end of the file.
When editing a man page make sure to update the $Date$ and $Release$.
For translating man pages we use po4a.
To add a new translation just add your language code to doc/man/po/LINGUAS and re-run cmake. Then run make update-manpage-<language code> from the toplevel build/ directory. The same command has to be used to update the .po file later when a .pod was changed.
To build the manpage without compiling darktable you can run make manpages to generate all of them or just make manpage-<language code> for one of the translations. The result will be in build/doc/man/*.1 for the untranslated man pages and build/doc/man/<language code>/*.1 for the translated ones. You can look at them by pointing man to the file, for example man build/doc/man/<language code>/darktable.1.