Liking this game a lot. My knowledge of British slang is pretty good, Spanish language countries varies a LOT and only know a few variations, Japanese and Russian non-existent, best I can do is read a menu using my alphabet and their words. Bellend ... is that really unisex? Very rarely encountered it, took way too much time screwing around on bike tour, and never got farther north than York because of time of year. My understanding was that (literally) it meant dickhead.
That's pretty much what it means, yeah. It can generally be used the same way any generic insult is used, where it isn't usually used to actually refer to the technical meaning of the term. Kinda like calling someone an asshole is almost never a case of calling them a literal anus.
It's not as common as it used to be, but does still get used. The main reason I used it in this case is because it's very much uniquely British, and felt like a decent way of accentuating the fact that Cassie isn't American. Couldn't use twat for that, which is another go-to, since Americans apparently do say that (Albeit, more like twought rather than twat.)
I can't personally attest to the accuracy of the Russian slang usage (Which doesn't come up often), but I get help with that stuff from my Russian translator, so I'd assume it to be reasonably accurate. For Japanese (And Indonesian), it's all just google translate at the moment, but I recently got in contact with a Japanese translator, who has offered to make the Japanese that shows up in the game more organic, so the Japanese should soon be native accurate. Not so important to the actual game, but I like to get details like that right where possible.