Complex sentence: To make the local inhabitants--especially Khmer and Chinese--understand French, colonists taught French to the locals, although the locals taught them their own language and some Chinese spoken variants, such as Teochew and Cantonese.
Split into simpler sentences: To make the local inhabitants--especially Khmer and Chinese--understand French, colonists taught French to the locals. The locals also taught them their Khmer, their own language, and some Chinese spoken variants, such as Teochew and Cantonese.