Hi there!
So, what you need to do is:
Download any font you want and install it (@edit - just use google to find a font that you like)
Use one of three ways to create font picture:
Online option:
OTC font create tool (https://egzot.github.io/otclient-font-create-tool/)
GIMP option (you'd need to download GIMP too):
otclient/tools/gimp-bitmap-generator at master · edubart/otclient (https://github.com/edubart/otclient/tree/master/tools/gimp-bitmap-generator)
OTClient font creator (https://otland.net/threads/otclient-font-creator.257389/)
cp1250 is font type for east/central EU so you need to pick this whenever there's an option to choose between cp1252 and 1250.
If you generate picture with font and name it polishFont.png, you'd need to put it into:
OTClient/data/fonts
& then replace all places in OtClient in OTClient/modules where you can see "Font: <name of the font>" to the "Font: polishFont".
Please note that there's also .otfont required file for your font. You may copy existing .otfont file & adjust it as you need.
Just to let you know, there'll be a lot of work to make the fonts look good.
@edit2
also, you may replace existing .png pictures in OTClient/data/fonts with the same fonts but generated for cp1250 that include polish sings, i.e. download verdana font, generate .png pic of verdana (cp1250) with polish signs & replace.
There's also a number of different formating for fonts - antialised, rounded, monochrome - and different font sizes. If you want the text looking good in the client, you'd need to manually round the letters, adjust it looks etc..
@edit3
Sorry for writing so chaotic xD but im at work & trying to help at the same time.
So, Tibia's official client used to have only one specific font in it. The official client possibly is still using only one font
However, OTClient uses a number of different fonts (or the same font but differently formated). The point is that creating one file 'polishfont.pnt' & 'polishfont.otfont' may not be enough to make it look good. You may create a replacement for each font file that you see in data/fonts