As far as I know, the author (HeyJake) MODERNIZER does not do localization ... maybe in the future ...
For non-English GUIs, the default font is Arial.
If you want the same font as in the English version, then you need to fix these files from the NOLF2.REZ \ INTERFACE \ FONTS archive
FONT_HELP.PCX
FONT_LARGE_0.PCX
FONT_MED_0.PCX
FONT_SMALL_0.PCX
FONT_TITLE.PCX
HUD_CHOOSER.PCX
MSGFONTBACK.PCX
MSGFONTFORE.PCX
NUMBERFONTBACK.PCX
NUMBERFONTFORE.PCX
FONT_AIR.PCX
You must add Polish letters to these files so that the total number of characters remains unchanged.
The problem is that the Polish alphabet has more letters than the English.
If you do Polish localization and ignore this problem, then all the "non-English" characters will be lost.
For example, you can add Polish letters only in uppercase, and lowercase letters in English. Then make a conversion table.
Unfortunately, I can no longer help you, because I do not create translations, but use ready-made ones.