The adjective belga - meaning Belgian - can be confusing, because it functions differently from other words used to describe nationality, such as:
italiano, italiana / italiani, italiane (Italian)
americano, americana / americani, americane (American)
russo, russa / russi, russe (Russian)
spagnolo, spagnola / spagnoli, spagnole (Spanish)
brasiliano, brasiliana / brasiliani, brasiliane (Brazilian)
The adjective belga is both the masculine and feminine form. This adjective also has an irregular masculine plural:
belgi
(not belghi)
The forms of belga are:
belga (m. and f.singular)
belgi (m. plural)
belghe (f. plural)
Belga in English is Belgian; Belgium in Italian is Belgio.