John Silence is an agent with British Intelligence.
He is not only one of their operatives, he is, going by the nickname the narrator uses several times in his adventures, the King of Spies. This rather heady praise is joined by a couple references to his being a "master-spy" which certainly sounds accurate considering the facts of one adventure.
In that instance he shows that he is extremely daring, or perhaps reckless or both, as he brazenly infiltrates a German base impersonating one of their officers. To pull this off, he would have to have not only a native speaker's command of the language, he would need to be totally conversant in body language and demeanor as well.
Then we find out he is able, when a British pilot he has broken out of imprisonment and then stolen a plane for the pilot to use to get both of them to freedom, is shot and cannot continue to operate the craft, Silence without hesitation takes over the task.