Which ISO format should I use to store a user's language code?
Should I use ISO 639-1 (2-letter abbreviation) or ISO 639-2 (3 letter abbrv) to store a user's language code? Both are official standards, but which is the de facto standard in the development community? I think ISO 639-1 would be easier to remember, and is probably more popular for that reason, but thats just a guess.
The site I'm building will have a separate site for the US, Brazil, Russia, China, & the UK.
Answer
You should use IETF language tags because they are already used for HTTP/HTML/XML and many other technologies. They are based on several standards including the ISO-639 collection (yes language, region and culture selection are not so simple to define).
I wrote a more detailed article regarding the proper language code selection and usage. The idea is to use the simplest/shorter ISO-639-1 codes and specify more only for special cases. Inside the article there are codes for ~30 most used languages with reasons why I consider one alternative better than another.
In case you want to skip reading the entire article here is a short list of language codes (not to be confused with country codes): ar, cs, da, de, el, en, en-gb, es, fr, fi, he, hu, it, ja, ko, nb, nl, pl, pt, pt-pt, ro, ru, sv, tr, uk, zh, zh-hant
The following points may not be obvious but should be borne in mind:
enis used foren-us- American English, and for British English is useden-gbptis used forpt-br, and notpt-ptwitch has much less speakerszhis used instead ofzh-hans,zh-CN,...zh-hant(Traditional Chinese) is used instead of more specific codes likezh-hant-TWorzh-TW
You can find more explanations inside the article.