In due course, we should install the relevant ISO 2- and 3-lettercodes as the enumerated possible values . . .
Source
<complexType name="shortLocalizedStringType"><annotation><documentation>A short string-valued text designed to be able to fit into a single varchar type field in a database.</documentation></annotation><simpleContent><extension base="womlcore:shortStringType"><attribute ref="xml:lang" use="required"/></extension></simpleContent></complexType>