yes, the more colloquial and 'rough around the edges' type of talk is usually reserved for men in Japanese. if a woman refers to herself as 'boku' for example, that is seen as inappropriate, but it is certainly not a swearword.
you see the culture is different. women and men have a different relationship in Japanese society.
In the west it is a power play, with feminism and sexism etc. In Japan, it is not so much. yes there are strong independent women who do things that are unexpected and take on traditional male roles, but it doesn't happen in quite the same way as it does in the west.
also, yes shizu and agito may have learned english swearwords, but what you need to be aware of is that they will not understand the true 'bite' of the words. if they do use the words, they are most certainly likely to use them inappropriately, either too often or in the wrong social situation. Because they don't have the social context that we have, the experience of growing up and being told the words were 'taboo' and then we used the words among our friends because we were 'naughty' and 'cool' at the same time... A Native Japanese is unlikely to be able to intuitively understand swearing in the way we do.
with regards to translation... ã“ã®é‡ŽéƒŽ, kono yarou, could be translated as 'this asshole', however it depends on the context of the situation, the translator would choose an appropriate term. in the west it is appropriate to swear at someone who was being a real asshole but in Japanese it would not be appropriate... not that such a word exists anyway. ah.. it's hard to explain. there is an inherent respect in Japanese society that means a 'swearword' is somewhat impossible. In fact the term 'kono yarou' would most accurately mean 'this wild-child'. but can you imagine a translator using that phrase in a scene that depicts a fierce fight? it's a translators liberty.