The first years of the National Curriculum in England removed teaching of a lot of grammar and language structure in English (as you can probably tell from this sentence). This meant that when starting to learn a second language at 11, no one knew or understood how their first language worked. The second language was an entirely new set of ideas that no one met outside the 2 hrs of lessons a week, so there were no examples to encourage practice or learning. The assumption in the content and approach of the second language lessons was that it could simply be bolted on to a full understanding of structure and working of the first language, which didn't exist.
Anyone struggling with English was excused a second language as early as possible. Everyone else was left in permanent confused frustration and dropped the second language as quickly as possible, replacing it with a subject they could succeed at which offered better grades and opportunities.
As an adult, learning a second language is still seen as impossible, especially to a standard that allows it to be useful, but the impact of not understanding the first language is still visible: my PhD was proofread by someone two years older than me, who had been taught language structure, who found a mistake about every second line.
Oh, yes, that would be a great super-power. As a native English-speaker, I have a massive advantage. However, trying to learn a language as an adult (Swedish) and ... oh, boy, it is HARD.
Technically, I have high school-level qualifications in French and Spanish. In reality, I got through that almost entirely due to my love of history, biology and fantasy books (and thus my large English vocabulary). You can guess your way through a lot of high school French if you know, e.g. not only the English words 'work' and 'job' but also 'travail'.
My manager and I visit Germany at least annually. When the brexit vote happened, we sat sadly and quietly, asking each other, "How are we meant to learn German before December?!" His point was that as two registered professionals we'd need a much higher level of fluency to top-up training and function in an equivalent role. The whole thing was overwhelming.
I’m trying to teach German (an increasingly abandoned second foreign language) in Scottish schools where teaching of grammatical structures in English was also abandoned long-ago. Classes of entirely mixed ability pupils, born speaking the most widely-spoken and understood language for a large part of the world they live in... It’s an uphill struggle to convince the majority of its value.