"but high voltage is not my expertise, and everytime that I work with something above 5V I got cautious "

So better not do what your schematic showed. Whether the same ground symbol is because of a drafting package limitation or otherwise,it sure is asking for trouble. ( Resistors have an upper limit on the voltage they can drop safely - checked it out ? )

The golden rule is to first isolate the mains - more so when you have no rule of polarization on your Brazilian plugs. A good quality step down transformer is the way to go - more so when you have finicky fellows like ADCs down the line. The space saving by eliminating a small step down transformer is not worth the trouble.

Raghu