To "intervene" in a lawsuit means to seek to become a party in a lawsuit that is already underway (rather than starting your own lawsuit from scratch). Usually, when you intervene in a lawsuit, it is to assert claims against one or more of the existing parties to the lawsuit. A "complaint in intervention" is the way you assert those claims. It is in many ways the same as any other complaint, although it might contain additional allegations explaining why the party believes he is entitled to intervene rather than bring a separate action. I don't know the Spanish word for it, but perhaps my explanation will help.