When to use -Ites / Ians / Ish / An / Ni / Ese / Elsh / Er [duplicate]
It’s not a matter of mood or environment, since you can’t change these endings at will. Foreign learners have to learn each one as they encounter them. Whether a citizen of a country and the adjective derived from the countrty's name end in –ish or –(i)an (or something else) may be largely a matter of historical accident. It may be relevant that the –ish suffix has a Germanic origin, the –(i)an one a Latin one.