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Are you asking cuz you're shocked quality is so good you couldn't tell it was fan translated or are you asking cuz you didn't know fan translations were and still is a thing? It's not just with manga, it's with manhwa and manhua too. They were all fan translated at one point, until officials get released then those get posted instead.
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Those fan translators usually form scanlation groups. Or sometimes it's just one person but usually they DO know the language. You've ever seen a recruitment ad page in the beginning of the manga/hwa asking for translators? Cleaners etc? Those are scan groups looking for more individuals to volunteer. They work together to make sure the raw material looks good for them to translate.
It's the ones that look unclean and translations dont make sense that are the google translators. Now those don't understand the language and they just churn out bad quality stuff.
The manga I’m reading is apparently fan translated—how do you even manage to do that