I have heard that it is more energy efficient to set air conditioner at a temperature of 26 celsius. Is it true?

It's a little more efficient, yes, because the difference between the outdoor temperature and the indoor temperature is lower, so the AC unit doesn't have to work as hard to 'lift' the heat to outdoors.  But more importantly the cooling load is lower too, for the same reason, the difference between outdoor and indoor temperatures is lower, so the unit can maintain the setpoint more easily, and will cycle off and run less.  So both of those factors will make your electrical bill lower, but you might drip sweat on it as you read it because 26 degrees is higher than what most folks consider 'comfortable'.