Question about Xiaomi Smart Humidifier 2

No, not necessary, it comes with an anti-limescale cartridge.

Nein, normales Wasser

This is not necessary. Of course, low-scaling water is good for any appliance, but it is not a prerequisite.

If the water is very calcareous, it is advisable to use filtered water, as with coffee water. This reduces the calcification of the appliance...