From looking around I have seen that Lauren S. Hissrich, the show runner, has given two reasons why she went with Jaskier over Dandelion.
To match the author's intent
She has mentioned on Twitter back in June 2018 when initially questioned over it that is was to match the author's intent. It's quite fun reading the tweets and her replies to the reasons some people give for why they don't like it.
@LHissrich: Geralt is Geralt in every translation. So is Yennefer. Jaskier is... all over the place. I’m taking him back to his roots, as the author intended.
@LHissrich: Jaskier is the original name in the books. It was translated into English, later, as Dandelion.
Twitter, 7:49 am Jun 23, 2018
For a bit of an aside Jaskier when translated from Polish into English actually means buttercup, the original translator felt that wasn't quite fitting for Jaskier's personality and so changed it to Dandelion. This is what Lauren is referring to above.
@Smashycomman: In the books: Dandelion. In the games: Dandelion. In the comics: Dandelion. In the Polish novels: Jaskier. Are you producing this show in Polish? No? THEN WHY WOULD YOU NAME HIM JASKIER?!?
@vonschirz: Exactly. From my point of view iťs sad and nonsense. JASKIER has got no linguistic base nor meaning in English.
@LHissrich: But... neither does Yennefer. It’s not an English name. Nor is Geralt.
@LHissrich: True. I get that. I guess in terms of adaptations go, I just always want to protect the author’s intent as much as I can, including the proper names he specifically chose.
@LHissrich: For what it’s worth, I LOVE the name Jaskier. It does sound romantic and foppish and delightful to my ear.
Twitter, 7:17 am 23 Jun 2018
She also gives an extra reason in the last tweet there, she loves the name Jaskier and presumably thinks it matches the character better than Dandelion.
The pronunciation issue
If the audiobooks are anything to go by Dandelion is not pronounced like the flower but rather "Dan-dill-ion". This appears to have been at least part of the reason to go back to the character's original name.
"We call him Jaskier. Yeah, absolutely. Jaskier is 100% in the show," Hissrich said. "It's interesting, because people ask about Jaskier all the time and why we went with his original name. It is funny, part of it was because when I read the books I read it as Dandelion (the flower), and then I listened to the audiobooks. Dandelion (pronounced Dan-dill-ion), how would I get that?"
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