Yea in America, "butt buddies" has a homosexual implication and sometimes used pejoratively. I would use a different term in English.

In US English, I’ve always understood it to mean guys who are attached at the hip / spend a lot of time together/ to the extent they could be mistaken as being in s romantic relationship / BFFs. I felt it’s appropriate but odd used in this example. Not bad for ESL though.

That said, just because I grew up in US and my butt buddies are here doesn’t mean they’re good cofounders. I’m regularly the only entrepreneurial person in my various friend groups.

It's incorrect translation. The correct one is "trouser friends" because they are so close they can borrow each other's shorts/trousers

Haven't heard that one. 'Blood brothers' might be close. (In case you're not familiar, it's an ironic phrase, doesn't mean that the 'brothers' are related at all, just very close: https://www.urbandictionary.com/define.php?term=Blood%20Brot...)

Or, I suppose, 'BFF', but perhaps not even teenage girls can bring themselves to use that seriously today (I suspect it's 'so last year').