I agree we should say Capital in English as that is just the translation. Calling it Das Kapital isn't incorrect though as that is the original title... I think any menace you interpret from the original title is from your own personal biases (or those around you/culture). The words are German and Germans are the bad guys in a bunch of US-centric movies. Plus like 150 years and a really strong 80 years of "Das Kapital is the commie Bible!" or whatever type shit. That's all additional cultural and personal bias though. It doesn't change the title of the book and anyone "in the know" knows Capital = Das Kapital.

Anyway, I can read French and a decent amount of Spanish. All I can say about the article "das" is it's very likely just one of those quirks of language. Maybe a native German speaker can say further, but there's just a lot of things in languages that can be translated directly, but don't make sense to be translated, so those doing the translations modify descriptions, wording, etc. to more fully and correctly capture the original meaning. Calling the book "The Capital" wouldn't necessarily be wrong, but we tend to care less about articles in English than some other languages. Even within different English dialects some speakers drop articles. One that sticks out to me all the time is British speakers will say "I went to hospital." Americans will say "I went to THE hospital" (or the indefinite article "a hospital"). Not an article, but Brits don't usually say "Back at my (or the) university" they just say "back at uni(versity)."

It's just a style thing ultimately, I think. Marx wasn't writing about the capital of his friend. He wasn't writing about capital within a single nation. He was writing about Capital with a capital (goddamnit) C. The big daddy Capital. The entire concept of circulation, commodities, trade, the history of money, labor, ownership. In that sense, The Capital sort of makes sense, but still just an awkward title I suppose