Thanks for quick answer!
https://ibb.co/et9TrH
Quality check did the trick it found all the books containing letter ł. I will doublecheck with another letter to see if it will make same amount of matches.
@jackie_w yep I have tried both on my inkpad2 and calibre and I suspect that my friend selected wrong coding or something like this and it "ate" all Polish letters. (it comonly happens in the world of internet and I am quite used to it but reading a book like this is a pain especially when Laska means staff and Łaska means mercy)
To my 2nd question:
Lets say I have 20 books by Karol May.
All of them have a proper title in the title box and Karol May in author (no metadata anywhere else) when I Ctrl+D them I get as a result Karol May in the title box and some dude that wrote his bio in author.
Edit:
Okay I have yet another problem.
The quality check did find all the books using PL letters and it displayed it as
marked:epub_search_text
How to display only the books that didn't pass the search?
Edit2:
I found out that I just needed to write marked:false. . .