If you are using a simple outlet tester you could get a false positive. You should always use something that draws some decent current, like a lamp. Here is a related story. I unplug the electric clothes dry recently while cleaning the ducting. My wife plugged it back in afterward. From then on, although the drum turned fine, the heat was intermittent. After checking a couple other things, I pushed the plug all the way in. Apparently, the motor ran on 120V, where the heater coil ran on 220V. Needless to say, I checked the plug and outlet for burns.
Look for light bulbs that are rated for 230-400 volts. By which I mean LED, the only technology worth spending money on in this day and age. They use electronic switching power supplies, which chop line voltage into a much lower voltage. By nature they can take a variety of voltages. You just need one that's built well enough to take 400V.
If your expensive electronics are multi-voltage, but can't handle 400V, then I would get a heavy step-down transformer, and configure it for 230V in, and 120V out. When the system faults toward 400V, the "120V" side will increase in proportion, but it won't go above 240V.
Losing neutral
What's happening is you are losing the "neutral" wire and the it's floating around, depending on the various loads on each leg. More loads pull the neutral toward that leg's extreme.
Euro power is 3 phase "Y" (3 legs with neutral in the middle) and your apartment gets 1 or 2 legs. If you remember enough high school geometry to draw a 120 degree Y shape with 230V legs, connect the corners and you get a triangle with 400V on a side. The corners are fixed and the neutral is being dragged around inside that triangle, making your power go higher or lower depending on which corner you are at.
That is not normal. The Hungarians who say that it is -- do they live in your same building? If so, it's a problem with the building neutral and the landlord oughta just fix it. If it's in the same city or region, then the power company is terrible.
Best Answer
It's hard to get breakers to trip in the sequence you want. Generally there are four ways a breaker can possibly trip.
Magnetic overload - a massive overcurrent resulting in an instant trip. Which breaker trips first will be totally random.
Thermal overload - this is a gentler overcurrent that will eventually overheat the wires. The breaker heats about as fast as the wires do, and trips before they get too hot. If two breakers are of a significantly different rating, the smaller one should trip first. If they are close, it comes down to manufacturing tolerance.
Ground fault (GFCI aka RCD) - all of them which see the ground fault will trip.
Arc fault (AFCI) - like the GFCIs all which hear the arc fault will trip. If you hooked a line-voltage speaker to the line, you'd hear mostly 50/60 Hz hum, and literally the sound of arcing. That is what the breakers are listening for. Breakers can hear crosstalk from other breakers, and so more than just the affected breakers may trip.