In the payment request:

Defines the language of the payment page (ISO 639-1 standard).

If the field has not been sent or is empty, the payment page will be shown in the language of the buyer's browser.

In the response:

Returns the value specified in the form if the buyer has not changed the language of the payment page.

Returns the language selected by the buyer if the buyer has changed it by clicking on a different flag.

Input and output field, returned in the response (IPN and Return URL).