Changing the system language back to english and once more to polish doesn't change them.
Try deleting the offending bookmarks and then recreate them.
To delete: Choose "Remove" from bookmark context menu, or just drag them out of the window.
To recreate: Show the home folder and then choose "Bookmark" from the folder context menu, or drag the folder onto the sidebar.
When first created they will appear at the bottom of the list (above Trash) - just drag into the desired position.
This assumes you have not changed the default user special directories (ie they are still in your home folder). If you have changed them, just navigate to the appropriate places to recreate the bookmarks.
You could also try renaming them using the bookmark context menu.