Would the translation still stand if you substituted prepositions, i.e. "with" instead of "to"?
It would make more sense to do so. It doesn't really make sense to be chatting "to" anyone. Chatting is something you do "with" someone, because both are speaking.
"Talk to" and "speak to" are, in my opinion, more natural than "talk with" and "speak with", which sound a bit more serious
Not being a native English speaker, I am used to "talk to" and "speak with".
I agree, but it seams like the primary translation has been changed from "chatting to" to "talking to", which my comment was directed at. Besides, there do exist situations where it would be natural to say "speak with" instead of "speak to". It depends on whether you are the only one speaking, or it is more of a two way communication. "Dad is speaking with my teacher" seems to be an example where this could work.
It seems like it would also be correct to say, "Dad is conversing with my teacher," though it marked that translation as incorrect.
Yes, kusema would be best translated as to say, and kuongea would be translated as to speak or to talk. At least this is what my Kenyan friends told me. It might be different in Tanzania. I'm not so familiar with the word kuzungumza, but I guess it's more in the sense of conversing or discussing.
The basic meaning is "together with", which sometimes can be translated as "and" and sometimes not.