Yes, I've picked up a good deal of idioms and slag over the years. Denmark is a small country, and Danish is a small and insignificant language. The bulk of Danes understand English, and most can even speak it adequately. If you have interests (like reading fantasy/sf or rpg) or work that involve English, or have attended university your English skills are bound to be better. So it's not really comparable to a native English speaker learning a second language, our daily exposure is massive. Also, I learned English by living in USA for a year over 30 years ago.
And as for not just writing Order of Hermes Lore, that is an occupational hazard of writing under the tutelage of Mr David Chart.