¿Cuál es tu/su nacionalidad? tu = informal su = formal
"¿Cuál es tu nacionalidad?", or "¿Cuál es su nacionalidad?", depending on whether you want to use the familiar or friendly form (for "you"), or the formal form.
¿Dé qué nacionalidad es (él)?The él is optional; if you use it than the sentence emphasizes that is is his nationality, but it is unnecessary.
The Guatemalan people refer to themselves as Guatemaltecos.
Nicaraguense