[susan53]

Possible yes, but it would need a convincing context to show that I didn't know when the specific decision moment (which is an event with a specific time) was :

Matteo made a complete fool of himself when he gave that speech. But I know that he's decided to improve his English since then.

This implies that he's already made the decision - but not necessarily that he's put it into action. For that you'd need to say eg :
After that disastrous speech, Matteo decided to take English lessons. And he's improved a lot since then.


So as always it depends on the context, how he speaker perceives the event and what exactly they want to express. If you leave those out of your examples it's very difficult to give a definite answer.