This is a more theoretical post than we are used to. From Linglish.net comes this explanation about the pattern in the formation of nationality adjectives.
Remember the eight basic forms of nationality adjectives:
-ian (Italian, Norwegian)
-ean (Chilean, Korean)
-an (American, Mexican)
-ese (Chinese, Japanese)
-er (Icelander, New Zealander)
-ic (Icelandic, Greenlandic)
-ish (English, Irish)
-i (Iraqi, Pakistani)
But read the Linglish.net to find out the recurring patterns in the way these adjectives are attached to different countries.
Also, do not forget to use the map below as a reference to get to know which suffix is used with specific countries.