After logging into Webmail, the default Polish language can be changed to English. In order to change language of user interface to English follow the steps showed below:
In the upper right corner, find the gear icon and click on it.
You will see Ustawienia podstawowe option. Then go to język section and select English (US) instead of Polski.
After changing the settings it is required to log in again. To do that, go to drop-down menu available after clicking the profile picture image and select Wyloguj (logout).
After logging again into the Webamil, the English version of interface will be available.