Hasn't a large proportion of the population of Holland been able to pickup British Television Broadcasts since the 1950s that might also account for the uptick.

Ainoa A, Albert B, Arturo B, and Augusto R.
Seems statistically unlikely!

I knew a woman who was a native Mandarin speaker who always turned on English subtitles even when watching a program where the spoken language was English. Note that she had college level proficiency in both spoken and written English.

Makes perfect sense. I grew up half my life in Germany and I hated how everything was dubbed into German (I remember watching Top Gun in America and then again in Germany, I almost started laughing in the theater it was so bad). But yes, my friends from Austria, not so good at English. My friends from Denmark, amazing at it...

This has been well known, at least in Sweden, for a very long time. And I would guess that if you would have made these polls before the internet then the differences would be even higher. I think the internet will eventually be an equalizer. But I might be wrong as much of the social media have bias algorithms isolating interests, specifically content in certain language in this case.

The idea of using the self-reported ability to hold a conversation is not really a valid basis to get a scientific result. There may also be significant differences in how the people report, and what they consider to be a conversational level.

I love how the characters in Norsemen speak in English

The Test of English as a Foreign Language (TOEFL) is the more important marker in the paper. Sorry if that’s not so clear in the story.

There is a additional influence, in my opinion: the size of the particular 'language world' as an economic factor supporting dubbing; the smaller 'language worlds' opting for more economic subtitling. There are, according to Wikipedia, almost 100 million people worldwide who have German as a first language. It is the most widely spoken native language in the European Union. When you compare this with the number of native speakers of Swedish (9.6M), Danish (5.5M), Norwegian (5.2M) or Dutch (22M), it is clearly a larger market.