Can be German as Romain Bouchard said, can be English, can be Dutch (mostly as Sander (name)); Zander, Sander, and Xander are abbreviations of Alexander. Xan Fielding was born in 1919, and the oldest Xander listed under Xander was Xander Berkeley, born 1955. But the name was popularised through Xander Harris of Buffy.
As a *surname*, Zander (surname) appears to be only German.