Except you talk about countries like India, Pakistan, and Bangladesh, where it literally was the language of a brutal colonizing force for over 200 years.

I wonder how far that exposure argument holds, globally, though. It's definitely omnipresent in the western world, but I couldn't say anything beyond that.

As a point of comparison, Romanian has a pretty straight-forward pronunciation, but has a lot of complexities that I wouldn't know how to explain to an outsider why they are like that other than "because it sounds correct"

It's easy because of the lack of genders and the exposure, but there are still a bunch of complicated rules especially in regards to pronunciation.

Obviously I have a personal preference for English if the world were to pick one lingua franca but grammatically with its cases, tenses and progressive forms it's relatively complex to learn, actually.

Please correct me if someone has more experience, especially in Japanese.

Korean is with a few exceptions. Hindi has male and female but only denoted through verb endings. Japanese has no grammatical gender iirc but makes a lot of social distinctions between male and female.