I can answer for a small part of Europe, namely Sweden.

The first secondary language of Sweden was not French, but German. Sweden had long and intimate economic, acadmic and religious ties to Germany, dating back as far as Medieval times, when much of the burgher class were either German in origin, or spoke Low German in order to communicate with the merchants and traders of the Hansaetic League. It was not until the Spring semester of 1944 that English overtook German as the most popular secondary language taken at Swedish schools. That said, English had been in ascendancy for some time, and German likewise in decline before English overtook German.

The Sweded in general took a dim view of nazi German persecution of union men, social democrats and socialists and the April 1940 German invasion of the "brethren people" of Denmark and Norway was something the Swedish general opinion would never forgive the Germans.

The waning of German power and the waxing of Western Allied power and fortune as the war progressed certainly helped as well. The Swedes were quietly preparing for the post-war world.

French however remained the primary language of diplomatic and economic contacts outside Germany before, during and shortly after ww2. When Sweden and Finland held staff talks about a possible mutual defence of Åland and perhaps even further co-operation, Colonel Airo, who headed the Finnish delegation refused to speak Swedish (despite having a good command of the language) and the talks were conducted in French in a rather icy atmosphere.

As the Western Allies won the war and USA ascended to superpower status and the French influence in the world decreased, Swedish diplomats started switching from using French to using English when conducting diplomatic talks. Still, when Sweden formally entered the UN as a member in 1946, the Swedish foreign minister at the time, Östen Undén held his speech at the ceremony in New York entirely in French, despite the audience being 99% made up of Americans, most (if not all) who did not speak French and could thus not understand a single word of Undén's speech.

SCB, the Swedish national bureau of statistics continued to produce its yearly book of Swedish statistics in Swedish and French from 1914 until 1951. The 1952 edition was published in Swedish and English instead, probably marking the last time Swedish authorities used French at a big scale.