[1] He was one of the central figures of the Albanian language studies and founding member and director of the Albanian Institute of History and Linguistics, and later Institute of Linguistics and Literature (Alb: Instituti i Gjuhësisë dhe Letërsisë).

An active participant of the National Liberation Movement during World War II, he worked in the post-war press-organs where he succeeded in management positions though at a young age.

[3] Upon his return in Albania he started working in the Institute of Sciences (Alb: Instituti i Shkencave), a precursor of the Academy of Sciences of Albania, and later in the Faculty of History and Philology of the newly founded University of Tirana where he would later reach the position of the dean.

In 1958, he was elected as Director of the Institute of History and Linguistics (Alb: Instituti i Historisë dhe i Gjuhësisë) of Tirana, and after it was restructured, the Institute of Linguistics and Literature (Alb: Instituti i Gjuhësisë dhe i Letërsisë) until 1990.

[2][4] Another field where Kostallari contributed was the history of the formation and development of the literary standard of the Albanian language, and its structural and functional characteristics.