Title pretty much says it all, but from my limited knowledge, the Norman invasion of 1066 did not see the French force Norman French on the English, Napoleon’s regime did not require the (for instance) Holy Roman Empire to speak French, and whilst the Warsaw Pact nations often had to teach Russian, they did so as a foreign language whilst Hungarian and so on remained the official language of coerced nations.
I’m not denying the impact of an invader’s language - English clearly owes a huge amount to French, as well as Norse and many others - but what I want to know is, were there ever any active plans to replace English with German had Operation Sealion been a success?
You are right: often the simple military conquest of a territory does not lead to the substitution of the language, which is usually the result of a longer process of assimilation of the conquered into the culture of the victors.
However, for the specific case of the Second World War and Nazism, a different discourse must be made: the genocidal violence of Nazism planned (and in some cases succeeded) the disappearance of entire ethnic groups and their languages. For example, Yiddish, the primary language of Eastern Jews spoken by millions, has now all but disappeared from Europe.
In much of Western Europe, and then the UK, "would you all speak German" can be understood to mean that German and not English would become the predominant foreign language, at least in German-influenced Europe, as you rightly have remembered happened in Soviet Europe. There was no particular Nazi plan to destroy English or French or Spanish or Italian culture, except the destruction of all "perverted" art.
However, a separate discussion must be made for the other continental Germanic languages. No concrete plans were ever put in place against the Dutch, Flemings, Danes and Norwegians. However, it should be noted that in Hitler's "Table Talks" with his closest hierarchs (long evening monologues in which the dictator chatted about every topic that came to his mind), the idea of annexing all populations directly to the Reich surrounding Germanic. All these were in fact, in the Nazi vision, fully Aryan and therefore easy to Germanize, by imposing the German language on them among other things. At other times their deportation to Eastern Europe was suggested to Germanize the lands conquered from the Slavs and at the same time repopulate their countries of origin with Germans from Germany. All this remained in the form of imaginative after-dinner talks.
But in Eastern Europe it was different: the Nazis planned and implemented as far as they could the ethnic replacement of all of Polish and Soviet Europe. At the end of the thirtieth-year "Generalplan Ost, all the territory from Poland to the Urals was to be exclusively of German language, "race" and culture. The previous inhabitants would be exterminated or deported, some immediately (Jews) as perceived as irreducible enemies for the German domination, who after (Slavs) having served the new Germanic masters as slaves.In particular the Poles were to cease to exist as a nation, in a manner no different from the Jews, and Poland to become a thing of the past like Gaul or Avaria So, to answer your question, yes: Polish would be banned by the physical destruction of its speakers. This was carried out during the war, resulting in the deaths of over two million non-Jewish Poles and the selective extermination of artists, clergymen, politicians, academics, professors and administrators.