default language is described rather than defined in 1.4

[azaroth42]

Currently in 1.4:

The default language is set in the context using the @language key whose value MUST be a string representing a [BCP47] language code or null.

But that doesn't define what the default language is, just how to set it.

Propose (or similar):

The default language is the language used when a string does not have a language associated with it directly. It can be set in the context [...]