I cannot type some special characters that require multiple keyboards key (such as "ś")

Such special letters require to press multiple keys on the keyboard, and those keys may be linked with some shortcuts of the translation interface. Try to fix this by checking your shortcuts 

Here is how to check your shortcuts:

Note: You may not find the exact shortcut in the list, but one that is very close of the key combination that you need to type the letter. Change it, and try again to type the letter.

This settings is user-based, meaning that everyone having the problem must change the shortcut.