How would you say "about a sandwich"?
I wrote "rozmawiamy" and it was accepted.
That should be right. „Mówić” implies more a one way communication, „rozmawiać” is two way.
That was really very useful, Vengir, thank you, you deserved a lingot !
Why do you use the locative case in this sentence?
As noted above, "o" takes locative.
'Mowimy o kanapkami' was wrong...?
Yup, it's wrong, wrong case. "kanapkami" is Instrumental. Like "Jesteśmy kanapkami" (We are sandwiches) if we were starring in "Sausage Party".
"o" = "about" takes Locative.