If you do not have a point of reference to test a pole voltage indicator, you will not get a voltage reading, even if one or all conductors are live at the same voltage. If you have a known path to earth or neutral, you can test using one of them to achieve a voltage value. If none of the existing cores at the switch do not offer an alternative path from live, you will only get 0V, Even if they are live and carrying 230V.
I would try a Dutch forum, they tend to follow German wiring methods not British, and in Britain we tend to use the ceiling rose as a junction box, when in rest of EU they tend to use the switch as a junction box. So better to ask in a non British forum.
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