The main consideration you need to keep in mind is Voltage. It does not need to be a power transistor if the load is only a port pin on the arduino.
The easiest way to interface in this case is with an optocoupler. A AC optocoupler exists. You just need to create a current limiting resistor value for the voltage seen on the diode. STandard open collector input would be the way you interface to the input pin... (with internal pullup enabled)
So again... your collector current will be very small so there is no need for a high power transistor.
For simplicity sake... You could use: https://www.mouser.com/ProductDetail/Broadcom-Avago/HCPL-3700/ for interfacing.
The Input side would have something like a 2W 2.7K Resistor inline on each AC leg.