Per the title, Charles Boyer was the French lead actor who starred in many Hollywood movies from the 1930's to the 1970's. If, in contemporary times, he might be most famous for his performance in Gaslight (1944) with Ingrid Bergman, he was already long famous as a romantic hero in exquisite films such as Love Affair (1939) (later remade as the lesser, yet somehow better known: An Affair to Remember (1957)), and the charming Hold Back the Dawn (1941) with Olivia de Havilland. In addition, Boyer had also co-starred opposite Rita Hayworth in one of the most compelling short stories in the multi-plot Tales of Manhattan (1942), which boasted an all-star cast.
For the purposes of the joke, Charles Boyer had to speak a factually erroneous statement, per the script: he said there is a French expression, "balle de vis," which he translated as meaning "screwball." In fact, there is no such expression in France; it is just a literal French translation of the two words "screw" (vis) and "ball" (balle). The correct word, in French, is "vis à bille," but has nothing to do with the aforementioned American slang, as there is no equal translation in the French. They did this for simplification.