Nesha learned the language from the priests and priestesses at the temple she was raised in.  I think of them, at least some of them, as being language scholars, especially in the local languages.  Sort of like missionaries who learn to speak the language of the tribes they minister to.  -grins.-  And there's no need to make it so he can speak Mwangi, if you don't want to.  Nesha could teach it to him, or Chula might allow you to take it free, since it makes sense that he would know it.