Working for Robespierre: help with the title and author!

I'm trying to find the title and author of a book my father read as a child in Slovenia sometime around 1950. The title was "V sluzbi pri trinogu" which translates to "In the service of a despot/tyrant".

The central character was a member of the Swiss guard at the time of the storming of the Bastille and arrest of the king and queen. He later became a spy, infiltrating Robespierre's office, pretending to be deaf/dumb to gain employment with him. In one scene there is nearby gunfire and he has to bite his lip to prevent blowing his cover.

Other figures from the time are mentioned in the book, including Marat and Danton. Not sure when the book was written, but I'm assuming it to have been very well known to have been translated from French or English into Slovenian. (At least, it seems more likely that it was written in France or England rather than in Eastern Europe).