RFC 3066 spells out the rules for creating language tags. Basically, for use in XHTML and HTML this boils down to four possibilities:

1. Use an ISO 639 language code. For example, en to represent English.

2. Use an ISO 639 language code followed by an ISO 3166 country code, to indicate a regional variant of a language. For example, en-GB represents British English, as opposed to, say, US English.

3. Use a complete tag as registered with IANA. For example, there is a registered tag for en-scouse, which is a dialect of British English used in the Liverpool area.

4. Use a user-defined tag. For example, I might use x-ishidic to refer to a language I had made up myself.

Country codes are often upper-cased, but this is only by convention - it is not a requirement.

The HTML specification still says that language values are described by RFC 1766, but this was obsoleted by RFC 3066 and an erratum is planned for the HTML specification to reflect that.