I passed Latin and German proficiencies back in the day, mainly because I had a decent memory for vocabulary and was fast with a dictionary. I learned neither as a spoken language. I do not pretend to any expertise in linguistics.
That said, my layman’s response when this comes up in conversation is that English is an easy language to learn to speak semi-passably; varieties of pigeon English seem to crop up spontaneously, almost like an invasive weed, everywhere from remote Pacific islands to sub-Saharan Africa to Appalachia to urban ghettos.
English is an great linguistic stew agglomerated through waves of conquest of the British Isles amplified by centuries of colonial and commercial expansion. Declensions and conjugations ... dustbin of history. Hard rules of structure ... dustbin of history. Vocabulary ... completely open. Etc.
We are left with an amalgam that lends itself to improvisation but that is so open that it cannot really be “mastered.” How do you “master” something with so few rules? As it stands now, it is the world’s best language for humor, for irony, for wild creativity, for indirect and hidden meanings ... and for misunderstandings among even fluent English speakers, let alone the pigeon English speakers.
Using English with precision requires extra care. And understanding one’s audience is a high art. I don’t think any other major world language has this character — though I wouldn’t bet against something from India coming close.
Currently, I enjoy using Duolingo daily. German, Welsh, Spanish, Italian, Gaelic, Irish, Japanese. German gets the most use as I don't want to lose the original proficiency. I finished the Welsh course. It is since been enhanced, so there is room for another round to grow vocabulary. The Scots Gaelic voice synthesis is not great. Sentences are spoken with discrete words "pasted" together. Words spoken by males and females of different ages. By comparison, the German and Welsh synthesis are very good.