>You can't really ascertain how good I am at speaking, for an obvious example.

That comma, gives you away. The commas (and punctuation in general) in English are rather special in a way they convey the spoken language rather than designated rules.

Edit: As for the full immersion when I was learning English (as kid, 5th grade or so), I recall visualizing the words (letters) in my head while speaking. Certain mistakes like than <> then, it's <> it, their<>there etc. are unlikely to happen while writing due to the way language was initially perceived (and b/c I leaned if-then much earlier). Still, esp. with English I'd not consider translating anything unless my spoken version of it was good enough.