What is Switch Contact Rating?
The contact rating is the voltage/current that the switch can repeatedly connect and interrupt without degrading it's design life-cycle operation count.

Take the ratings as the maximum safe operational ratings of a switch.
Yes, a 240VAC 5A rated switch will operate just fine on 127 VAC, but up to 5 Amps.

The DC rating is almost always lower because of the ability of switch to quench the arc when opening a DC circuit.