BTW: What is the difference what the COUNTRY code and the LANGUAGE code affects?
In your case, you should use a language code since you want to set a language. ConTeXt has used different sets of codes in the past, but we now try to follow IETF recommendation "Tag for Identifying Languages" (http://tools.ietf.org/html/bcp47), because it's the only one that can be precise enough for our needs (at least among the tagging systems I know of). For example, you can distinguish between British English and American English by appending a country code to the language code (hence "en-gb" and "en-us", respectively). Using a country code to identify a language is generally a bad idea and should be discouraged, since that's not what they're meant for, and it can lead to confusion: for example, you could use the code "uk" to identify English as spoken in the United Kingdom, but that's actually the language code for Ukrainian, which has been an actual problem for ConTeXt in the post (in addition to that, "uk" is not even the proper language code for the United Kingdom: it's "gb", as written above; the reason why "uk" is used as a DNS top-level domain is not clear and has lead, alas, to even more confusion). The authorities that decide upon language and country codes are different committees of the ISO; the ISO standard for language codes is ISO 639 (with different parts), and the one for country codes is ISO 3166 (again, with different parts; the two-letter codes of ISO 3166-1 are generally rather well-known because they're the ones being used for DNS top-level domains -- with some exceptions, see above). Arthur