List of all valid Accept-Language header values

I am looking for a list of all valid (hopefully standardized) Accept-Language values.

I have searched high and low for a list of valid Accept-Language header values and the best I can find is this Mozilla article which only lists a few. It also doesn’t seem that these Accept-Language values are standardized as part of any type of ISO specification anywhere.

So I ask: does this list of valid values exist, and where is it hidden?