WP All Import on "[Plugin: Import any XML or CSV File to WordPress] How to fix the [Accept-Language] error while importing xml/json from url?"

This data would be stored in the page's HTTP header. This is something that you would probably configure with your theme or a multilingual plugin like WPML, but I'm not sure. Unless you have a way to store this header info in the database (in a custom field or something) there isn't any way to import this data with WP All Import.