@T-W-Jomtien0
The primary (default) language is the language used when the listing was created.
Useally you create a listing on the local Airbnb website (In your case the Polish one: www.airbnb.pl ) with an account also created on that website.
You can add extra languages, but to my knowledge you can not make another language the default language. Also not all info in a listing can be created as "multi language".
Please note guest useally will see the listing in their own language, if that language is added. Otherwise a translated version is provided.