Had the French in Paris started to learn German before their liberation in 1944?

Based on some of the numbers given in quotes of @LаngLаngС answer:

• Yes, about one half percent (0.5%) more French were learning German in 1942

Population Paris (city proper) 1936: 2,829,746
Population France 1931, 1946, 1950: 41,524,000, 40,125,230, 41,647,258

The Berlitz language school increased its enrollments from 939 adult students of German in 1939, to 7,920 in 1941; the German Institute in Paris from a few hundreds in the 1930s, to 15,000 by 1942, mostly taught by French instructors.
...

• 939 (0.033%) to 15000 (0.53%)
  • 99.47% were not learning German in 1942 in Paris.

Altogether, some 30,000 students took courses at the German Institute; the majority were adult members of the urban middle class and the liberal professions, with a total of about 100,000 French people learning German during the Occupation.

_{— Omer Bartov: "Mirrors of Destruction: War, Genocide, and Modern Identity", Oxford University Press: Oxford, New York, 2000, p 65.}

• 100.000 (0.249%) – based on population France 1946
  • 99.751% were not learning German in France during the Occupation.

Numbers are relative and must be brought into context.

The statement: 'one half percent more French were learning German' gives a different impression than a total of about 100,000 French people learning German during the Occupation.

The second may imply that that is a huge portion, when in reality it is not.

The French language was not in danger of becoming extinct during this time, just as there was no threat that the French people desired to assimilate themselves based on these numbers.

Sources:

• Demographics of Paris - Wikipedia
• Demographics of France - Wikipedia

OK. Let's be sticklish.

Q Had the French in Paris started to learn German before their liberation in 1944?

Yes. Before 1944. The French 'started' teaching German in their own schools looong ago. Before 1914 German language education was more popular than English. After that war, popularity declined somewhat, with English replacing German at top spot. But after the French defeat in 1940 and then during the occupation the desire and necessity for learning German increased again quite a bit.
The numbers of German learners peaked in 1942 for French school curricula and also for people voluntarily enrolling in German-led institutes. Despite resistance members urging their compatriots to always deny having any second language capabilities when facing German soldiers.

Any form of a first language attrition, or forgetting their mother tongue, in this short period of time is of course absurd. No one in France unlearned French, and certainly not in favour of German. Germans didn't prohibit use of French, and quite a few Germans tried to learn more or better French themselves.
German wasn't replacing French in France. But knowledge of that language did increase in that time frame for the general population, as the sheer number of those learning German by volition or out of need is documented as being somewhat higher than before 1939.

Since we are accustomed to blatant lies as well as hyperbole from twitterers…

Whether or not this man trumped a tweet correctly or not is left to judge for the reader:

While all this activity was going on, increasing numbers of French children and adults began learning German. The Berlitz language school increased its enrollments from 939 adult students of German in 1939, to 7,920 in 1941; the German Institute in Paris from a few hundreds in the 1930s, to 15,000 by 1942, mostly taught by French instructors. Altogether, some 30,000 students took courses at the German Institute; the majority were adult members of the urban middle class and the liberal professions, with a total of about 100,000 French people learning German during the Occupation.
This in turn reflected the accommodation of the intellectual and academic elites. Institutions of higher education, ranging from the Collège de France to practically all universities, rapidly and voluntarily purged themselves of their Jewish members, without any registered public protests by the professors. The prominent historian Lucien Febvre demanded the resignation of Marc Bloch, the Jewish coeditor and co- founder of their journal Annales, so as to save it from being banned. While Bloch joined the Resistance and was executed by the Germans, Febvre continued to edit the journal — which in no way promoted resistance — from the safety of his country house.

_{If you see a calculation stating: "100,000 (0.249%) – based on population France 1946 ☛ 99.751% were not learning German in France during the Occupation". Then it really should read: because of the occupation alone instead of a few hundreds indeed tens of thousands flocked into the fascist learning facilities ☝︎ and thus 0.25% of the entire population preferred to learn fascistic German instead of 'just the language'.}

Similarly, such respectable French publishers as Gallimard purged their lists of Jewish authors and turned to publishing texts by German racial ideologues and French collaborators. Nor did many renown French writers desist from having their works published during the Occupation, including Louis Aragon, Georges Bataille, Simone de Beauvoir, Maurice Blanchot, Albert Camus, Paul Claudel, Marguerite Duras, Paul Éluard, François Mauriac, Jean Paulhan, Romain Rolland, Jean-Paul Sartre, and Paul Valéry. Marcel Aymé wrote for the fascist antisemitic journal Je suis partout, Mauriac contributed to the collaborationist Nouvelle Revue Française, Camus cut out a chapter on Franz Kafka from Le Mythe de Sisyphe to have it published in 1942, Antoine de Saint-Exupéry paid a quasi-official visit to Germany to promote the translations of his books there, and Aragon excised references to Heinrich Heine and the Dreyfus Affair from his writings to facilitate their publication in Germany. Other artists followed suit. Jean Cocteau expressed admiration for Hitler, Abel Gance made a friendly visit to wartime Germany, and Pierre Fournier gave concerts in the Reich. That some of these men and women eventually ended up in the Resistance only highlights the moral confusion and the depth of accommodation that characterized the French intelligentsia in the first years of the Occupation.
_{— Omer Bartov: "Mirrors of Destruction: War, Genocide, and Modern Identity", Oxford University Press: Oxford, New York, 2000, p 65.}

_{If detractors deduce the above to be 'a puffed up piece' based on nothing more than circumstantial theoretical reasoning for one very minor point of the argument: According to his biographies (ex Schiff) Saint-Exupery did go to Germany, a few times, at the behest of mainly Otto Abetz, and said explicitly and repeatedly that he wasn't interested in any ideologies, not caring for any nazi thought, just expressing some disgust at what they did at the Führerschule he visited. Only the timeframe Bartov seems to suggest here is phrased perhaps a bit sloppily in that one point: it is not chronologically exact.}

Bartov quotes this number from Philippe Burrin: "France under the Germans, Collaboration and Compromise, New Press: New York, 1996, p299–305.
Repeatedly:

A striking example of French accommodation was the expansion of German language instruction. The numbers of French schoolchildren learning German rose dramatically, especially in Paris. The Berlitz language school had 939 adult students of German in 1939, but 7,920 in 1941. Some 30,000 people took courses at the German Institute, most of them adult members of the urban middle class and the liberal professions. A total of about 100,000 French people were learning German during the occupation, their numbers reaching a peak in 1942 and then dropping as the prospects of a German victory diminished.
Intellectual and academic elites were also very much part of this process.
_{— Review: The Proof of Ignominy: Vichy France's Past and Presence, Contemporary European History, Vol. 7, No. 1 (Mar., 1998), pp. 107–131}

An assessment that gives some numbers, but apparently implies very little:

Burrin’s concept of accommodation uses broad, contextual analysis to evaluate decisions made by various social groups including the Catholic Church, captains of industry, intellectuals, and regular French men and women who struggled to survive during the Occupation. Did French men and women who served a German cliente`le, manufactured products for the German war effort, or simply learned to speak German sympathize with the ideological goals of the Nazi regime, support resistance efforts, or just try to make some easy money? Without passing judgement, Burrin suggests that many people balanced personal ideals against the necessities of life and accommodated some German demands without necessarily endorsing Nazi goals.
_{— Thomas J. Laub: "After The Fall. German Policy In Occupied France, 1940 – 1944", Oxford University Press: Oxford, New York, 2010, p8.}

How this was attempted to be organised from the German side at the top is through offers of friendship, published propaganda pro germanophilia and propaganda against the 'old schools' of German studies in France, embodied by scholars like Andler and Vermeil. The former indeed always a staunch anti-German who did not really advance his subject that much but supplied political assistance for French politics towards its eastern border. The latter however was of a younger generation and known to promote peace through friendship or at least reconcilation with Germany after World War I. But this attitude changed after 1933 for some reason. But this younger generation was nevertheless the lever the Germans found the most useful to influence. Less with force, but with kind words, and a lot of money. The German Institute in Paris being the main coordinator for these efforts.

Curiously, while the occupational policies are usually described as being to get much harsher after the defeat of the Germans became ever more evident when reading reports from the Eastern front, the political efforts went the other way around; moving from repression to an outwardly attempt at more and friendlier cooperation. Not in the least to win over more French volunteers, for example for the 'fight against Bolshevism' and the like. For how honest these thoughts for 'a shared Europe' coming down from persons like Himmler and Ribbentrop a comment seems unneeded? At least at the time Hitler thought so and ignored them.
_{— Eckard Michels: "Das Deutsche Institut in Paris 1940-1944: ein Beitrag zu den deutsch-französischen Kulturbeziehungen und zur auswärtigen Kulturpolitik des Dritten Reiches", Franz Steiner Verlag, 1993.}

_{— Martin Mauthner: "Otto Abetz and His Paris Acolytes: French Writers Who Flirted with Fascism, 1930-1945", Sussex Academic Press, 2016.}

_{— Roland Ray: "Annäherung an Frankreich im Dienste Hitlers?: Otto Abetz und die deutsche Frankreichpolitik 1930–1942", Oldenbourg Verlag, 2009. (gbooks)}

_{— Bernard Trouillet: "Das deutsch-französische Verhältnis im Spiegel von Kultur und Sprache", Beltz, 1981.}

So of course: when one army invades a country with a different language, there is a certain incentive to learn the language of the invader, if you do not already know it. Almost equally, the invading army will have to up their language skills. Therefore, the original tweet would have to be amended that "until the time the Americans came along, the Germans were learning French in Paris"?

When the Americans went to Vietnam one side started to learn French and Vietnamese, the other started to add English to their repertoire as well. When the German army invaded Russia, they had interpreters with them to translate from and into Russian. When the Russians approached Berlin they had bi-lingual people in company. When the French occupied parts of Germany under Napoleon the rate of learning French in Germany increased. This is always going both ways. Contact necessitates communication. For that you need a common ground and that is language.

Why this stirs any controversy seems quite beyond reasonable thinking.

If there should still be any need for 'more perspective', given the hard numbers shown above that prove at least that even the officially registered language courses show a modest but significant increase for 'learning German language', which naturally do not include self-learners or pidgin/kitchen German acquisition by the locals — and also not any regular school tuition.

Those numbers from language learning at a regular school would be much higher. The numbers presented here above show only those not acquiring German in French schools — but strictly those entering voluntarily into 'enrollment in German institutions to learn the language' (Whether privately run, like Berlitz language school, or a German propaganda dissemination outfit local German Institute etc, not any lycée).

If we look at the numbers of attendees at the German Institute alone, an enrollment increase from "a few hundreds to 15000" at its peak means a roughly 30-fold increase in that single overtly fascist service provider.

To get a glimpse for the increased interest in private tuition:

„Les écoles de langues sont prises d’assaut. En 1939, l’école Berlitz avait 939 élèves d’allemand et 2470 élèves d’anglais, à l’automne 1941, les premiers sont 7920 et les seconds 625. Certains Français écrivent à l’administration militaire pour proposer à des officiers un échange de cours de langue. D’autres cherchent par les petites annonces un professeur à domicile; les plus exigeants spécifient : ‘aryen et si possible d’origine allemande’. [Mais] l’intérêt pour l’allemand déborde des bancs d’école. Il serait instructif de faire le recens*m*nt des cours qu’organisent toutes sortes d’institutions, des chambres de commerces à la SNCF, par souci de faciliter le travail en commun.“
_{— Philippe Burrin: "La France à l’heure allemande 1940-1944", Seuil: Paris, 1995. (p303–310)}

In regular secondary schools learning German was always second place after English from the First World War on. It took first place before. In numbers for school tuition we see a trend of decline from 1933–1951, the percentages being below 30% and reaching a low in 1950 at 21%. But again for 1941 a number of 29.8% and for the peak in 1942 while English declined very slightly we see for German another increase to 31.4%.
_{(– Frank Günther Spohr: "Deutschunterricht in Frankreich und Französischunterricht in Deutschland. Eine Bestandsaufnahme zur Lage zweier Schulfremdsprachen im allgemeinbildenden Sekundarschulbereich", Thesis, Ruhr-Universität Bochum, 1999.)}

But Burrin also lists in contrast to the just cited numbers a figure of 2/3 of all pupils learning German during the occupation, indeed reversing the pre-war situation in relation to English. Concerning these statistics It might be interpreted as a very good sign that while 100,000 people went to the Instituts allemande to foster their own economic advancement when accommodating for an apparently occupation de longue duree, only around 1000 of them ever used the attached libraries and only 10 books related "the Jewish question" went on loan there.
_{(Burrin, p309. Back of the envelope calculations for 30–70% of a part or total student population learning German in relation to absolute French demographics of the 1940s may eventually appear elsewhere on this page. Perhaps then aded with even at least 'seemingly exact' numbers from other sources.}

_{They will not change the simple fact: that occupation lead to a significant increase for 'learning German' in France.)}

To get an impression on "the other side": It might therefore be interesting to read what the former French trade ministry worker Jean Texcier wrote in his Les Conseils à l’Occupé (1940, PDF), a leaflet soon to be spread around resistance style, with advice for the French on how to collaborate properly — and how not. It gives you this perspective:

2. They are winners. Be correct with them. But don't go, to make yourself seen, in front of their desires. Don't be in a hurry. They wouldn't know you, moreover, any better.
3. You don't know their language, or you've forgotten it. If one of them speaks to you in German, make a sign of ignorance and, without remorse, go on your way.
4. If he asks you in French, do not think you have to put him on the road yourself by giving him a bit of guidance. He is not a fellow traveller.
5. If he tries to talk to you in a café or restaurant, politely let him know that you are not at all interested in what he is going to say.
6. If he asks you for a light, hold out your cigarette. Never, since the most remote times, has anyone refused a light - not even their deadliest enemy.

Bonne occupation pour des occupés. (Wishing the occupied a nice occupation — is just one of a lot of possible translations…)

Slightly more of that in English in Advice for German-Occupied Nations, Holocaust and Human Behavior, Chapter 8, but the French original is a bitter-sweet and still entertaining read for itself. So go and learn French with it.

Since the German soldiers were often forced to learn by immersion themselves, even if an overall relatively rare and rudimentary linguistic competence was present. Quite a few soldiers came to France utterly unaware that a French language might exist, others tried to learn what ever they could.

From letters and diaries, three examples written during the 'good times' of occupation, by those willing to learn:

"I will not take French lessons with Frl. Reguin (the charming daughter of the blind musician), but with M. Laurent, with whom I can learn much more. He is unusually witty and surely would be behind bars with us long ago.

The phrase book has always been my secret wish, and the comrade who received the second one is Sergeant Jato. He is a vocational school director in Eschwege. He knows the French language much better than I do, but he reached for it with both hands. He wants to thank you very much. He said: "My God, your wife thinks of everything."

I am now staying with a farmer in the neighbourhood, who always provides me with the bike. The people are also otherwise very nice and friendly to us. I am often invited for dinner, I already get wine every day, from the own vineyards and I have an excellent bed. I often spend the evenings with the people and improve my French language skills. The man was a prisoner in the world war and can also speak a lot of German.

_{— Kerstin Wölki: "Krieg als Reise – Die Wahrnehmung Frankreichs durch deutsche Soldaten im Zweiten Weltkrieg", Master Thesis, Albert-Ludwigs-Universität, Freiburg, 2007. (5.2.4. Sprache: „Der Sprachführer ist schon immer mein heimlicher Wunsch gewesen“)}