I bought an air conditioner in Thailand rated 220v-240v, 50 Hz. What could happen if I use it in our country which uses 220v, 60 Hz?
The motor will try to run 20% faster than the unit is designed to run. Increasing the fan speed by 20% will theoretically increase the load torque by 44% and the load power by 73%. The compressor part of the load will theoretically increase by 20% torque and power. Operating the motor at 220 volts on 60 Hz will theoretically reduce the motor's torque capability by 30%.

Your best course of action would be to sell it and buy one that is suitable.

Edit:

Changing the pulleys to get the proper speed as Brian Drummond suggests may be a good solution.

Most likely, a single phase motor is used. It would likely be either a permanent split capacitor (PSC) motor or a capacitor-start, capacitor run motor (2 capacitors). That complicates the issue of proper voltage. The impedance of the coils increases with increased frequency and the impedance of the capacitors decreases. It is difficult to predict how that will affect the motor.

If the unit contains an inverter, that could potentially solve the whole problem. However, if that were the case, I would think that it would be labelled 50/60 Hz.