What happens if I use the 110v (US) electric toothbrush charger on 220v (South East Asia)
This will fry the magnetic coil from twice the excitation voltage and possible saturate the core and melt it.  try it out and post the Youtube video for proof of concept (j/k);)  It will be far more than twice the temperature rise when the inductance (reactance) drops to just the wire resistance. 

I suspect the small gauge wire inside will self-fuse.

It reminds me when in 1970 when my older brother borrowed a $5k portable tape recorder from CBC and smoked it on 230Vac during his trip thru Africa with a Land Rover.  ( He majored in Political Science not engineering)

added


  However consider the very low VA rating of this device, it would be  perfect candidate for a 2:1 stepdown voltage transformer inside a USA bathroom junction box and receptacle for shavers and toothbrushes alike that run off 120Vac.
  
  Although with the price of e-toothbrushes, it may be a thing of the past.