I have a problem adding new In-App purchase in iTunes Connect. The definitions gets rejected and "English" cell gets highlighted to yellow.
The publication language is Polish. When adding a new In-App purchase, there is no option to select Polish language.
How shall I write descriptions - in english or pick English and write in my national language?
The app store does not support Polish as localized language, therefore you should use English (or the most relevant language for your country) for the App Store. Inside your app you could use Polish if you prefer.