foo must be an ISO-639 language code

The value specified must adhere to the W3C format for language codes.

An example of a valid language code: en-us

Don't use the actual name of the language, like "English". Instead, use the 2- or 3-character language code, like "en" (ISO 639-1) or "eng" (ISO 639-2). (You can also specify a country code after it, like "en-us". This is optional, and the validator will ignore it.)

If you do include a country code, you need to separate the language code and the country code by a hyphen. "en-us" is valid; "en_us" is not.

The full set of language codes are defined by ISO-639 language codes.