I just find stuff like this fascinating as a former Special Forces soldier who speaks both French and German but am not a native speaker. In the movie, Inglorius Basterds, there is an incredible scene where this is hammered home. The Americans, posing as Germans, spoke flawless German but weren't raised in Germany. When asked if he wants a beer, the American signals three by raising his three fingers. A Gestapo officer immediately recognizes this mistake...a native German would signal three by raising the thumb and two fingers. Having lived and worked in Germany this is absolutely true. In the Army, to be considered 5/5 fluent on your language test, you have to have been born and raised in that country.
The Captain lifts three fingers up for the bartender to see, and orders “Drei Gläser” (three glasses). He used his index, middle, and ring finger. In this pivotal scene, the British spy outs himself simply by using the wrong fingers to order a drink.