No, I know of 4 countries that don't require them either: Austria, Netherlands, Czech Republic and Poland (unless you're going to a towered field). It's in the AIP.
Interesting. I don't know about the others, but the Dutch AIP states:
A flight plan shall be submitted prior to operating:
Any flight or portion thereof to be provided with air traffic control service;
Any IFR flight;
Any flight across the Amsterdam FIR boundary;
Any flight or portion thereof operating in the North Sea Area Amsterdam and North Sea area V (see ENR 2.2 paragraph 3 and chart ENR 6-2.5);
Any VFR flight operated in airspace class A, under special conditions (see ENR 1.2 paragraph 3.2).
A flight plan can be transmitted during flight by radio if the flight plan covers only part of the flight. This does not apply to flights of which parts are executed:
Within the Schiphol CTR;
In controlled airspace class A;
In controlled airspace class B, except if the flight is carried out with a glider;
In controlled airspace class C, above FL195.
I agree that the wording is a bit ambiguous, but "if the flight plan covers only part of the flight" does not sounds like an appropriate exception to me. If your flight is across a FIR boundary, then the whole flight is an international flight, and not "part of the flight". So I don't think the exception clause would apply here.
I fly from a controlled airport (Rotterdam) and in the AD section it specifically states that all flights need a FPL, so I'm not keeping track of this all too closely. But I know that in the past the requirement for FPLs when crossing a FIR boundary was more explicit, and specific exceptions were made for flight between a few uncontrolled airports close to the border. (Teuge-Stadtlohn for instance.)
Also, a few years ago we had a presentation from a Dutch Mil controller and with regards to FIR crossings he said something along the lines of "We can correlate your Mode-S return with the FPL so we know it's you. That fulfills the obligation of notifying the FIR crossing. So don't call us if all you want is to tell us you've crossed the FIR boundary. Only call us if you actually need something from us." That, again, would suggest the authorities expect a FPL for any international flight.