Purchase Order - Header (How to change language - Urgent)

Dear All,

In purchase order header, communication data tab i find a field called Language, which is basically defaulted from the vendor master when the PO is created. I would like to know how i can change the value in this field after the PO has been created. I need to change this for a few PO's for a specific reason, but notice that the field is greyed.

I tried MEMASSPO, but it is not having any effect on the PO.

Hope my problem is clear, await inputs.

Regards,

Vivek