Translation:Who were you talking to?
bIjatlhtaH = You are speaking.
DajatlhtaH = You are speaking it.
tlhIngan Hol DajatlhtaH = You are speaking Klingon.
nuq DajatlhtaH = What are you speaking?
'IvvaD bIjatlhtaH = To whom are you speaking?/Who are you speaking to?
IvvaD DajatlhtaH = To whom are you speaking it?/ Who are you speaking it to?
If something that you are speaking is specified or asked about, you must use Da-. If Da- is present but the thing being spoken isn't specified, the English translation should supply "it". If the thing being spoken isn't specified and there's no need for the listener to infer that it's relevant, use bI-. And so on through other verb prefixes such as ma-/wI-; jI-/vI-; and Su-/bo-.
Note that by placing the 'Iv after the verb (in the place of the gramatical subject), you are indicating that you are asking to know who is doing the speaking. However, this conflicts with the Da- prefix which indicates that "you" are the one speaking.
If we assume this was simple misplacement and move 'Iv before the verb ('Iv DajatlhtaH) we still have a problem. The grammatical object of jatlh is the thing spoken, so this sentence would translate as, "Whom are you speaking?"
To indicate the beneficiary or recipient of your speaking, the person you are speaking to, you must use -vaD: 'IvvaD DajatlhtaH "To whom are you speaking it." Note that the Da- prefix includes an object so even though we have not specified what is being spoken, we have clearly indicated that we have a particular thing in mind (and expect context to make it clear what "it" we are talking about). If we don't want to indicate a specific thing being spoken and just mean speaking in general, we can instead use the bI- prefix and we get the sentence at the top from this exercise.