In due course, we should install the relevant ISO 2- and 3-letter
codes as the enumerated possible values . . .
Use of this attribute is required.
Its form is unqualified.
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Base type of lang is language. There are several restrictions on the value. The schema processor collapses value's whitespace. The value must match "([a-zA-Z]{2}|[iI]-[a-zA-Z]+|[xX]-[a-zA-Z]{1,8})(-[a-zA-Z]{1,8})*".