You should have a look at Wakaama, rather than Leshan. It is a C implementation of LwM2M. See https://github.com/eclipse/wakaama . If you have any question on Wakaama, the developer’s mailing list is at https://dev.eclipse.org/mailman/listinfo/wakaama-dev
AFAIK there will be a Google Summer of Code project to help with porting LwM2M to RIOT (hopefully using Wakaama) so you might want to follow that too (a quick search in the archives of the mailing lists will give you more info on the people involved).