This test expects an error:

    <test-case name="format-integer-063">
        <description>Third argument as a string not in the form of a language code</description>
        <created by="Michael Kay, Saxonica" on="2012-12-07"/>
        <environment ref="empty"/>
        <test>format-integer(11, 'Ww', '@*!+%')</test>
        <result>
            <error code="FODF1310"/>
        </result>    
    </test-case>

but according to http://www.w3.org/TR/xpath-functions-30/#func-format-integer

"If the $lang argument is absent, or is set to an empty sequence, or is invalid, or is not a language supported by the implementation, then the number is formatted using the default language from the dynamic context."