This is a wonderful illustration of how, when it comes to language, there is only one hard-and-fast rule: UISS-IWC-MINS (Unless It Sounds Silly–In Which Case, Make It Not So)—or UISS, for short. The rest are all guidelines.
If you had to program an AI program with formulating the adjective from country names, it would look something like this:
- Start by tacking on, or replacing the last syllable with, the great Default English Adjectivifier—<ish>—especially if the country name ends with <land>:
Thus: Britain > British; Scotland > Scottish; Ireland > Irish; Finland > Finnish; Sweden > Swedish; Poland > Polish, Turkey > Turkish.
Unless It Sounds Silly–In Which Case, tweak to Make It Not So. Thus: England > English (not Engish); Wales > Welsh (not Walesish); France > French (not Francish); Spain > Spanish (not Spain-ish);
- If it ends with an <a>, tack on an <n>: America > American; Russia > Russian; Australia > Australian; etc.
UISS-IWC-MINS: e.g. by adding an <i> before the vowel—e.g. Canada > Canadian; Argentina > Argentinian. If that doesn’t work, try guideline #3.
If it ends with another vowel, tack on an <an>—e.g. Italy > Italian; Hungary > Hungarian.
- If the country’s name ends with <l>, <m>, or <n>—tack on, or replace the last syllable with <ese>: Portugal > Portuguese; Senegal > Senegalese; Lebanon > Lebanese; Sudan > Sudanese; Japan > Japanese; Taiwan > Taiwanese; Siam > Siamese. Heck, if it’s in the Far East or Southeast Asia, do this even if it ends with a vowel—e.g.: China > Chinese; Burma > Burmese; Java > Javanese
- If it’s Asian or African but ends with some other consonant, tack on an <i>—e.g. Israel > Israeli; Afghanistan > Afghani; Uzbekistan > Uzbeki; Pakistan > Pakistani.
- If none of the above work, consult the French for inspiration: e.g. norvège > Norwegian; suisse > Swiss (not Switzerish); grecque > Greek (not Greecy).
- If all options fail (i.e., they all sound silly)—just leave as is—e.g. New Zealand lamb—or avoid using an adjective, altogether (e.g. “It’s, er, from New Zealand”).†
For ease of reference, here’s a handy flowchart:
P.S. Carefully set aside and preserve all silly-sounding endings for Monty Python skits and other comedic purposes.