This is the 6233rd most frequent Polish word.
belg
"Belg" in Polish means "a Belgian" (a person from Belgium).
Here, 'Belg' is used as a noun denoting a person from Belgium, in nominative singular form.
Belg odwiedził naszą szkołę, aby opowiedzieć o swoim kraju.
The Belgian visited our school to talk about his country.
'Belga' (genitive singular form) specifies possession or origin, indicating the paintings belong to or are created by someone from Belgium.
Podziwialiśmy piękne obrazy autorstwa Belga.
We admired the beautiful paintings by the Belgian artist.
In this sentence, 'Belg' (nominative singular form) emphasizes the nationality of the subject's brother.