I've encountered the same bug and have a couple of additions to add in the specifics of when it happens since I was curious.

This bug only occurs in the English versions, I tested the other languages.

The bug only occurs after the quest has been officially completed. I talked to the NPC before completing the quest, and she would reply in English.

https://i.imgur.com/ksXmtCR.png
https://i.imgur.com/HDzReeo.png

Once it's complete though, she'll reply in Japanese.

https://i.imgur.com/T9XCMcv.png

Hope this can be of help :]