We are creating multi-language subsites on our website.
I would like to use the 2-letter language codes. Spanish and French are easy. They will get URLs like:
mydomain.com/es
mydomain.com/fr
but I run into a problem with Traditional and Simplified chinese. Are there standards for which 2 letter codes to use for these languages?
mydomain.com/zh
mydomain.com/?
@dkarp gives an excellent general answer. I will add some additional specifics regarding Chinese:
There are several countries where Chinese is the main written language. The major difference between them is whether they use simplified or traditional characters, but there are also minor regional differences (in vocabulary, etc). The standard way to distinguish these would be with a country code, e.g. zh_CN for mainland China, zh_SG for Singapore, zh_TW for Taiwan, or zh_HK for Hong Kong.
Mainland China and Singapore both use simplified characters, and the others use traditional characters. Since China and Taiwan are the two with the biggest populations, just zh_CN and zh_TW are often used to distinguish the simplified and traditional character versions of a website.
More technically correct but not commonly used in practice, however, would be to use zh_HANS for (generic) simplified Chinese characters, and zh_HANT for traditional Chinese characters, except for rare cases when it is meaningful to distinguish different countries.