To enable a new .po file, add its basename (in fact, the language code), to the first line of po4a.cfg:
[po4a_langs] de es fr pl uk
It is not crucial to sort the entries alphabetically, but do it anyway for better readability.
This is done in one step using the following simple command:
If there's something wrong with a .po file, the command will fail. In any case, have a look at the first line of po4a.cfg if all of the mentioned files are present.
To the man page authors: Please don't forget to add your new man page to po4a.cfg!