It may be that Polish characters don't EXIST in the text encoding this game uses, it returns a "?" for each unrecognized character.
To check if this is the case, look again at the file after you modified it just in case it has all Polish characters replaced with "?".
Keep in mind you can't change the game's encoding without major code changes, so if that's the case, you try and hijack other European characters the game does accept, and add Polish characters to their font graphics. Those are a graphic in the game with all possible characters in the game, if the character isn't there it simply doesn't exist and can't be summoned by the game.
If there's not enough characters to add all Polish characters, you might have to resort to extreme measures like making the game text all caps, and use the lower case characters for Polish.
You said you checked the Russian version. This is a very good start. The game was never intended for Russian markets, so all Russian glyphs were never in the game in the first place (at least Polish has some) which means they had to modify the font. Check which files were modified compared to the English version this game was based on (if it's PS1, the serial might help) and you'll find the font and text you need to modify, pretty soon.
Please let us know if you managed to find it.