The description element cannot have "xsi:nil" as an attribute.
The description element in XML instance can be substituted with other elements.
Other element cannot use description as substitution group head if
the element's type is derived by using one of the following methods : restriction.
In due course, we should install the relevant ISO 2- and 3-letter
codes as the enumerated possible values . . .
Use of this attribute is optional.
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})*".