Thx for your fast answer.
But I don’t have “/wp-content/languages/participants-database/participants-database-LOCALE.mo”.
I tried to find another folder “languages”, but with no result (I got only folder “languages” with language examples, *.po and *.mo files).
Screenshot below:
https://zapodaj.net/images/6bd2b932748a8.png
And weird thing is, that I have a polish translation (that I need for all texts) by default. I mean I don’t change anything. Most texts are polish (that I’m glad) but that “The file you tried to upload is too large..” is english.
Screenshot below:
https://zapodaj.net/images/fc3a45f71395b.png