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 !
So "mówimy o kanapkah" is like a sandwich conference where people give speeches; while "rozmawiamy o kanapkah" is people having a conversation about sandwiches?
If that is the case, "mówimy" is more suitable IMO (i get the impression "rozmawiamy" is more "talk" than "speak")
Mówimy is ambiguous, whereas rozmawiamy explicitly denotes an (interactive) conversation. So, yes, mówimy is more similar to speaking, but we also accept rozmawiamy here.
How would you say "about a sandwich"?
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.