"Anybody who is able to read and understand word Desi" is desi. That includes Indians, Pakistanis, Bangladeshis etc.
By the way, "Dais" is an Urdu word that means "Homeland" or "Watan".. the Arabic equivalent is "Watni" and English equivalent is "Native". In General sence, Desi means anybody who belongs to civilization of Indian Subcontinent.
Also the word "Desi" is most probably taken from Urdu because in Urdu the name for homeland is "Dais" while in Hindi it is "Daish". If that had been taken from Indian "Daish", that would've been "Deshi" but its "Desi" and that implies it is an Urdu term instead of Hindi.