This will be a brief enough and nit-picking response to only one example given here: William the Conqueror actually attempted to learn English unsuccessfully. The ‘French’ they spoke was not really French but a rural dialect now called Anglo-Norman. English was not outlawed, and the common people who spoke English did not expect to speak the same language as their masters anyways; they were used to nobles and kings who spoke Dutch (Canute) and French (Edward the Confessor) anyways. Anglo-Norman was spoken by the Norman conquerors not as a means to oppress the English population, whom they depended on, but as a matter of inertia, because that was simply what they spoke, and what their children spoke. It was not long (approximately a century) before even child nobles who spoke Anglo-Norman had to be schooled in the French of France before being sent off there for schooling, or else they would not be able to converse with the French. While being treated as a language fit only for common labor and conversation at first helped English to simplify its grammar (and Anglo-Norman did add some 10,000 words to English), the ruling Normans gradually came to see themselves as English rather than French, and would have known how to speak both Anglo-Norman and English. By the early 1400s, defense of the English language was being used by Henry V to rally troops against France itself.
Most of this I get from Bill Bryson’s excellent book “The Mother Tongue,” pages 53 to 58.