It's hot where I am too in a typically AC-less country.

I used to live on the Mediterranean in some crappy apartments with no AC and this is what we used to do to sleep well on hot nights:

Go to bed later. If you know it's gonna take a while to fall asleep, you might as well wear yourself out. This pairs nicely with a nap after work if you're the napping type. My best sleeps were during the day because I'd be so exhausted from not sleeping enough at night.
get rid of the duvet but keep the duvet sheet on the bed so at least you're snug underneath something but it's not as warm.
I'm echoing the advice of everyone else here to get a fan. Those things are great.
not specifically heat related, but if mosquitos are also keeping you up, get a cheap plug in repellant (you might be able to get them in the supermarket in NL?) so that a rare night's sleep in the heat isn't ruined by an insect bite.

Living in places that aren't designed for any sort of weather is gas. Six months ago I was almost bankrupt heating my place because it's not insulated properly. Now half the building is a sauna. We had a sweet spot in the middle for about six weeks I'd say.