It's called banter. You guys sound poisoned by overly literalist fan translations with stilted dialogue that you have to re-write in your head to sound like something a person would actually say.
Too liberal translations just do not read well. There are plenty very good translations and groups (see Henkyou no Roukishi for example) but this one is not very good. I'll wait for volume releases and read it raw because this is just dogshit.
As for the translation, the only question I'd have is how likely is it that the real dialog is actually using Japanese teen slang. If it is they I don't mind the translations attempts to sound modern, if it isn't then yeah, they could dial it down a bit. Either way, I'm not suffering physically from it so, whatever. Just glad it's back at all.
I think this translation reads exceptionally well. In fact, while reading
RuriDragon on MangaPlus, it occurred to me that there are few fan translations on MD that approach its quality. And
WaffleIroner is right about the reason.
Even when fan translations are good, they're typically far too literal. They capture the standard definitions of the original words, but fail to find a conversationally natural way to accurately express the speaker's meaning and tone in another language.
"Even if you say that," is a familiar example of bad translation. It attempts to capture a common Japanese phrase (そんな事言われても) but is awkward and borderline meaningless in English. In English, we'd instead say something like "even so," "nevertheless," or "that's easy for you to say."
I also agree with
TheRealAlpha2 that slang is appropriate only where the original text contains slang or conveys a comparably informal tone. But Ruri's slangy English dialogue suits her character well, so I'm not inclined to complain.