"This" means 'here and now'. If we already spoke about it it becomes "that". The Polish grammar for " to" and "tamto" doesn't apply to English usage. Therefore when in Polish you say 'już rozmawiałyśmy o tym' itbwould be much more likely to translate "tym" to "that" since you are referring to the past. That being said, of course it could also refer to "this". For example, maybe there is a printed list of topics which is " here and now".