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:

• en is used for en-us - American English, and for British English is used en-gb
• pt is used for pt-br, and not pt-pt witch has much less speakers
• zh is used instead of zh-hans, zh-CN,...
• zh-hant (Traditional Chinese) is used instead of more specific codes like zh-hant-TW or zh-TW

You can find more explanations inside the article.