Our Windows app is available in Polish. TO change the language, please right-click on the TD icon in the notifications area (system tray) and use the "Logout" option there.
Next, while you're on the login page, please click on the "English" link at the bottom and you'll see the same language selection options as on the web and Polish will be one of them.
The program Todoist on windows not everywhere is Polish material, eg a calendar month are in English.
We will soon improve our Polish translation and address this issue. Sorry for the inconvenience.