Go to standardXPath function: boolean lang (string)
Standard excerpt:
The lang function returns true or false depending on whether the language of the context node as specified by xml:lang attributes
is the same as or is a sublanguage of the language specified by the argument string. The language of the context node is determined
by the value of the xml:lang attribute on the context node, or, if the context node has no xml:lang attribute, by the value
of the xml:lang attribute on the nearest ancestor of the context node that has an xml:lang attribute. If there is no such
attribute, then lang returns false. If there is such an attribute, then lang returns true if the attribute value is equal
to the argument ignoring case, or if there is some suffix starting with - such that the attribute value is equal to the argument
ignoring that suffix of the attribute value and ignoring case.
Examples (xslt:xml): Overview
59:13
Interactive xlab: [xpath] [xsl:value-of] [xsl:template]