Garrett Coyle asks this question:
[Why is] the adjective form of U.S. state names … generally the same as their noun form, but the adjective form of foreign countries is generally different from their noun forms[?] So it’s “I’m from California” and “a question of California law,” but “she’s from France” and “a question of French law.” Is there any reason for the difference?
I don’t know the answer, and before the question was posed I wasn’t even conscious of the difference — though, of course, together with nearly all other lawyers, my speech and writing reflected that difference. A quick Westlaw query for “Californian law” in the ALLCASES database uncovered 10 cases, one of which used the term “Mexico-Californian laws,” and zero references to “France law” (except for a few references to the France Law Digest). But there are 1856 references to “French law,” and too many references to “California law” for Westlaw to accurately report (but I did find 693 references to “California law” in just the last two months).
So we know what’s standard usage, and that’s enough for practical purposes. But I share Mr. Coyle’s curiosity about why this is indeed standard usage.