What language did Nazi German occupying forces speak in France?

Partially, the German occupying forces are speaking English when working together with the French police, especially when no translators are at hand. Is this historically proven?

No, of course not. It's a movie! Just about anyone speaks English in Hollywood movies. No matter what.

Use your common sense. Speaking multiple languages is not common for most people. Foreign languages are taught in high school and above. Not very often on primary schools. The main stream western pupil went to high school and higher after WW1 and more so after WW2. My mum went to primary school only, just before WW2. As she wasn't going to high school, she would do an extra year in primary school. That was at that time the legal requirement in The Netherlands.

The Dutch, for example, are now known to be very good second language English speakers. Before WW2, that was definitely not so. The English proficiency is fairly recent, from the 70's onwards to today. We see the same in Scandinavia, who are also very good in English as a second language.

Now, neither the French nor the Germans are known to be linguistic masters. Why would French dockworkers and German military speak to each other in a third language? It simply makes no sense.

Good communications were quite difficult. The Germans would use German, of course. Which the dockworkers would not understand. One of them perhaps a bit, who would do the translation. A German officer who would speak some French would talk with and by him to tell the others what he wanted them to do.

Which languages were in use during the occupation?

That's what they did. They used the language(s) of the occupied territory together with German. In case of doubt the German original was what legally mattered.

• https://img.tweede-hands.net/pics/00/15/27/94/71/2c.jpg?508930cae2

Here a German announcement of the execution of resistance fighters in Maastricht, The Netherlands in German and Dutch.

La Rochelle is a port city. The people you describe seem to be either sailors themselves or working in professions were regular contact with sailors is required. German sailors usually spoke English when abroad in the relevant period. I would imagine the harbour police would also be able to communicate in English.

Never underestimate the ability of people to learn (a basic level of) a foreign language if they really have or want to.

Example of a German sailor being able to communicate in English with locals in Belize in the decade before WWI here. The author is a troubled kid from a middle class family and his English seems to be just so-so, but the other sailors seem to speak English just fine, without this being noteworthy in any way:

Wir drehten bei und warfen den Fischern, die in einer mir unverständlichen Sprache laut zu uns herüberschrien, eine Leine zu ... Einer von ihnen klomm die von uns über Bord gehängte Falltreppe empor und verhandelte an Deck sehr lebhaft mit dem Kapitän in englischer Sprache. Ich konnte nur einzelne Worte davon verstehen, aber Hermann verdolmetschte mir den Sinn seiner Rede. Der Mulatte erklärte, daß er Fischer wäre und uns für fünfzehn Dollar nach Belize bringen wollte. Allright! ...

[Ich lauschte] mit höchster Spannung auf das, was der Fischer von Belize erzählte. Freilich konnte ich bei seiner raschen Sprechweise nur wenig übersetzen, aber ich verstand zum Beispiel, daß Trinkwasser in Belize sehr teuer wäre und Früchte sehr billig, daß nur wenig große Schiffe dorthin kämen, dagegen aber viele kleinere.

Translation:

We turned and threw a rope to the fishermen, who were shouting to us in a language incomprehensible to me ... One of them climbed the ladder we had hung overboard and negotiated quite intensively with the captain in English. I only was able to understand single words, but Hermann [another sailor] described to me the gist of his speech: the mulatto said that he was a fisherman and would bring us to Belize for fifteen dollars. Alright! ...

[I listened] intently to what the fisherman had to say about Belize. Of course I could only translate little because he spoke so fast, but I understood for example that drinking water was supposedly very expensive in Belize and fruits very cheap, that only few big ships came to Belize but many small ones.