Jak King writes:
Are there rules in English for making adjectives from countries, or are the assignments random? I have found a number of standard adjectival endings (-ese, -(i)an, -ish, -i, -er). There are also some singularities (French, Greek, Monegasque) and some where the adjectival form is the same as the country name (Hong Kong, New Zealand).
How is this worked out, or who decides?
Those are excellent questions.
The answer to your last question, "Who decides?", is "no one". Or better, "everyone". Usage generally converges on a single answer in each case.
How is this worked out? Well, no one really knows, but one simple theory is presented here (with some additional background here). The basic idea is that if the members of a community start with a random distribution of distinct beliefs, and exhibit these beliefs in their behavior, and learn from ("accommodate to") one another by adjusting their beliefs in the direction of their experience, then the community converges to a shared state. (This is true as long as "belief" is viewed as a probability distribution over categorically-distinct alternatives.)
In any case, it's clear that language, like most other aspects of culture, is what Friedrich Hayek called a "grown" or "endogenous" or "spontaneous" order, rather than a "made" or "exogenous" or "artificial" order.
As for the specific question of adjectival forms of place names in English, there's some discussion in "The evolutionary psychology of irregular morphology", 4/10/2008.
And if you want more, there's "The theology of phonology", 1/2/2004; "All your base are belong to which lexical category?", 5/15/2004; "Who let the 'n' in?", 1/22/2006; "Chinian, not Chinese?", 1/26/2006 "'Democrat majority': Offensive but not ungrammatical", 1/31/2007; "More political morphology: Democrats, Great British, and Geese", 2/19/2007; "A map of adjectival forms of place names", 4/11/2008.