You indicated you went through the system preferences and ticked the required boxes, but didn't tell us what options you set. Here's what I did to set up my system for Polish fonts. I don't understand the language, but here are some of the characters that I suddenly could access.
ż ó ł ą Ż § Ł ć ń ś
1) Open the Language : Text Szstem Preference pane then click on the Input Sources button and select Polish.At the very top of the input sources list also make sure you put a check next to the 'Keyboard & Character Viewer' option. This will add a tiny menu bar option in the upper right corner of your display that allows you to quickly switch between the Polish and other language keyboards and languages. Just pull down the menu bar and select the 'Show Keyboard Viewer' option to display a virtual keyboard on your display that will allow you to enter the unique Polish character in the event you don't have a Polish keyboard.

2) If Polish is going to be the primary language then click on the Language options and drag Polish to the top of the list.
