Regarding Cortana, I don't think you can. Cortana is currently only available as a Beta in the US largely because international support simply isn't ready. I'm guessing this is one of those things that wasn't done (or its a bug).
That being said, you don't have to ask the forecast and then ask to convert it. At least for me, saying "What is the temperature tomorrow in Celsius" (or appending "in Celsius" to any weather requests immediately returns the response in Celsius.
Regarding the Calendar app, this is controlled via the phones "Regional Format", If you change the regional format to something else (like Canada) the Calendar app will show Celsius (at least on my device). This does not affect the availability of Cortana (so long as you leave Country/Region set to the United States).