Wow. I think of 15C as "comfortable if I'm outside doing stuff and wearing long sleeves", but inside, it's "put the heat on (to 18C) and put on a sweater". 25C inside is what we set the air conditioning to in the summer at night so it's easier to go to sleep, but in the day, 26.5 is fine. This is in the southeastern US, and I've converted all temperatures from how I normally think of them in F.

Also, my workplace keeps inside air conditioning much cooler than I would, around 20C, which is uncomfortably cold when I'm dressed for the summer weather (ranges from 27C to 38C).