If they're deriving any benefit from their story being hosted where it is - and it's not your place to judge what counts as "benefit", stuff like likes/kudos and reader engagement would count - then a translation, or any other substitute, posted anywhere else will decrease their benefit. No amount of credit or citations/links can reduce that effect to 0. If they think the translation provides more value to the world than they get from the extra engagement, they can give you permission. If they don't, it's not your call to make to say otherwise. Or you can just email them the translation so they can post it on their account, if you're really doing this for selfless purposes and not trying to gain reputation/intangible internet points off someone else's work.
Edit to add: also I have personal experience in scientific writing, where "citing someone doesn't mean you're not plagiarizing them" is an established standard. Not that normal citations are plagiarism, and even decent-sized quotes labeled as direct quotes with a citation are relatively standard acceptable practice, but if you steal too much of someone else's verbatim text and not as a quote, slapping a citation as a bandaid attempt won't actually save you.
You are confusing "plagiarism" and "copyright infringement".
Plagiarism is part of what are called "moral rights". Plagiarism is passing someone else's words off as your own, which is considered to steal the real author's recognition. Plagiarism is not actually against the law; it is against scientific etiquette and creative etiquette and may lose (a lot of) reputation within those communities, but you can't be sued for it. Plagiarism
requires deception; as long as you make it clear who said what, and your claimed division is actually correct, you're not committing plagiarism, because it's about credit, not money.
Copyright infringement is an
economically-based tort (that is, it's illegal; you can be sued over it). The purpose of copyright is to encourage the creation of new works by granting the author a monopoly and thus allowing them to charge high prices for access to that author's works. It's not about who gets the credit; it's a pure bribe. As such, posting someone else's copyrighted work without permission, even with attribution, is still copyright infringement.
(NB: I oppose copyright on the grounds that the deadweight losses and loss of mythopoeia outweigh the gains, particularly since the copyright industry has demonstrably been extremely willing and able to rent-seek rather than actually working for its keep. Patronage is enough, particularly with the Internet allowing decentralised patronage via Kickstarter/etc.)
Plagiarism and copyright infringement are different things with different rationales, even though they sometimes coincide. Don't use the language of one to refer to the other.