zosia

You shouldn't have changed that path name. It must end at Dictionaries.
Affinity will look below that path to find the dictionaries. Thus, with your change, publisher is looking into C:\ProgramData\Affinity\Common\1.0\Dictionaries\pl_PL and there it expects to find another folder named pl_PL.

Please, how do I make it to work inside the Publisher? I have followed this: 
Everything went OK, I restarted Publisher, but my language still isn't present there (Czech language (cs-CZ), Publisher 1.8.3., Windows 10). Thank you.
 
EDIT: Now it works well. I made a mistake and pointed to the cs-CZ directory itself. Later I changed the path so that now it ends in the 'Dictionaries' directory. Thank you so much for the tutorial.