My understanding is that most countries just don't bother; I once drove around North America on Danish plates; since European plates are much wider than North American style plates, none of their cameras could scan my plates; so camera-only toll roads were essentially free for me. I consider that it happens so rarely anyway, that they don't bother.

Similarly, I've been flashed for speeding in France, which does have cameras adjusted to my plates' size, but they also didn't bother sending a ticket. Germany - on the other hand - will send you a ticket, but since they allow Ö, Ü, etc. on their plates, their system can probably handle Æ, Ø and Å as well.

Edit: Obviously, they don't bother to a degree; severe infractions will obviously make local law enforcement do something, but it's a rather manual process. Most countries are signatures to a treaty, that recognises other countries' plates.