Well, in Poland we use normal English alphabet + these characters: ą, ć, ę, ł, ń, ó, ś, ź, ż, so I won't need to replace that many characters, and to edit the font I'd just need to add a simple line to existing characters.
Thank you all for your support, I didn't even thinked that people from all around the world (or at least Europe and America) would help me with a Polish translation.
EDIT:
Oh, wait, I can't extract it. I'm using Linux with Wine and when I open the HackingToolkit3DS.exe and choose correct options (at first I type cxi, then I type the name of the file, then I type y and it immediately says that it has completed extracting, but I can't see the extracted files anywhere. I've put the .cxi file to the directory where HackingToolkit3DS.exe is located. Am I really forced to use Windows? Or maybe I can extract it with other tools?
The .exe file that I've got is from this thread:
https://gbatemp.net/threads/tutorial-how-to-decrypt-extract-rebuild-3ds-roms.383055/