"We talk to them when they appear."
Translation:Vi pratar med dem när de dyker upp.
Old question, but will reply in case someone else is interested!
Talar is a little more formal than pratar generally speaking. I generally think of att prata as to talk and att tala as to speak in order to help differentiate them in my mind.
As in English, they are mainly interchangeable in meaning but you would be more inclined to use talar in a more formal setting eg speaking with a doctor or lawyer, and pratar more generally for talking to people.
I feel that dyker upp contains the connotation of surprise; they were not expected to arrive (yet), but showed up unexpectedly, whereas kommer fram is more of an announcement. In Dutch we have the verb opduiken/duiken op, which has sort of the same connotation. So, that's how I differentiate between the two, but I might be wrong...