Voltaire visited Frederick the Great as a French spy, something which he took no pains to conceal. Most of Europe then thought of Frederick the Great as a floofy monarch who didn't pose a threat to anyone because his claim to fame had been trying to run off with his boyfriend, and then getting caught. His father then threw Frederick the Great into prison, where the guards mocked Frederick for playing his flute and reading French literature.
Surely Frederick wouldn't catch on. The fact that most of Europe was pretty pissed at Voltaire for his satirical poetry, and the mission was pretty much given as a 'get-the-hell-out-of-my-country-if-you-won't-stop-writing-about-me' sort of trip by Louis XV did not daunt Voltaire either. Thus, he set off from France in high spirits, going so far as to grandly inform a sentry gaurding Westphalia, "I am Don Quixote!"
The sentry apparently did not speak any language other than German and, smiling and nodding, let Voltaire pass.