I believe 'speak with' is also correct although less common. 'Speak with' is closer to the swahili
If 'semeni' means speak to, why do you need na there? Wouldn't you be saying "Speak to with teacher"?
No. It depends on which type or form of English is used. KUSEMA: to speak. Seme-ni ( suffix -ni = you pl.). Semeni na ( na = to - U.K. English; with = U.S. English) a teacher. So, (You pl.) Speak TO a teacher (U.K. English) or (You pl.) Speak WITH a teacher (U.S. English). I hope that explains 'na' in this context...