Create readme.txt in your folder with polish language pack and put 'Polski'. Look at /usr/language/string/, if you see en-gb change your folder name to en-gb. Put language pack to /mmc/mmca1 and use mount:
mount --bind /mmc/mmca1/en-gb /usr/language/string/en-gb
Go to Setup -> Language and choose your language
If you want your own FW with polish language pack... you need unsbf, then unpack cramfs(*.bin), change what you want and merge all files to sbf.
Some month a go i have my own FW but I really to fear flash my phone using it ;P