Polish charecters, fonts in moder ui?
 

When I select
polish language
in modern UI, then I have indeed polish text on UI
elements (like buttons "Next" is "Dalej",
"Previous" is "Wstecz")

-but-

there is a promblem - the polish nationall characters
like "ą" "ę" "ć" "ó" are messed up - like if they where in
wrong encoding or something...

How to fix that?

I am using NSIS for linux/Debian (makensis) from Debian Testing

Are you sure you have the correct language for non-Unicode setting in the Regional and Language Options control panel? It must be set to Polish, NSIS installers are not Unicode enabled.