Complex sentence: Columba could not speak the language of the Picts whereas Moluag was fluent, which could explain why Moluag evangelized largely Pictish areas and Columba stayed within the sphere of Dál Riata influence.
Split into simpler sentences: Columba could not speak the language of the Picts whereas Moluag was fluent. This could explain why Moluag evangelized largely Pictish areas and Columba stayed within the sphere of Dál Riata influence.