Since the node argument is often calculated, we should allow empty:

fn:lang($testlang as xs:string?, $node as node()?) as xs:boolean?

Summary: This function tests whether the language of $node, or the context 
node if the second argument is omitted, as specified by xml:lang attributes is 
the same as, or is a sublanguage of, the language specified by $testlang. The 
language of the argument node, or the context node if the second argument is 
omitted, is determined by the value of the xml:lang attribute on the node, or, 
if the node has no such attribute, by the value of the xml:lang attribute on 
the nearest ancestor of the node that has an xml:lang attribute. If there is 
no such ancestor, then the function returns false

If the second argument is the empty sequence, the function returns the empty 
sequence. If the second argument is omitted and the context item is undefined 
an error is raised: [err:FONC0001]. If the context item is not a node an error 
is raised [err:XPTY0006]XP.