SQL is the prime language of interaction with MonetDB. However, it is continuously being extended with small and major code fragments that extend the functionality described in the SQL 2003 standard. It includes completely wrapped libraries, e.g. geos, or address non-core SQL.
BEWARE, as mentioned in the Mar2011 release notes, work on the Pathfinder compiler, which forms the basis of MonetDB XQuery, continues at The University of Tübingen.