Can I charge the 12" macbook with Nintendo switch charger?
I have tested on the 2015 MacBook 12". Works perfectly. Charges at 15.3V @ 1,9A = 28,7W. The MacBook 12" charger also works on the Switch.

USB-C and recent USB Power Delivery standards make this a fairly safe bet in terms of cross platform interoperability. As long as the charger or cable aren't knockoff, you should expect reasonable results mixing vendors. 

Reasonable is a 15 inch MacBook Pro clearly won't run and charge from a 30 watt adapter (whether Apple or Nintendo), but if powered off, it will slowly and safely charge itself at the rate the connected adapter provides.