Can a 220V relay be used to switch 110V appliances?
The 220 volt relay could be used to switch 110 volts.  However the contact current rating will not increase.  Contacts rated for 5 amperes will not be safe switching more than that, certainly not 10 amperes. The contacts carry the current when closed so the voltage is not the issue but the size and material of the contacts is important. Therefore do not exceed the contact current rating no matter what the value of the voltage being switched