No battery lasts for ever, so eventually it will need replacing. When the watch stops isa good indication that it is time to replace it; and when the second hand behaves erratically is an indication that an upcoming stoppage is about to happen.
When the battery is replaced, a good repairer should also re-seal the watch so that the waterproofness is maintained.
Do not go to the market stall to replace it! Do not go to one of the High Street shops that advertise battery replacement. Do go to a specialist watch repairer if you have one near you.
Alternatively, you can send it to one of the retailers advertising on line, or send it to the official Mondaine Service Centre. You will not need to speak French or German to do this, as they are located in Leicester! See the page on servicing under this same tab.