I don’t own/use a kindle but did a 2 minute search and found this promising fourm post comment from user Enterio www.mobileread.com/forums/showthread.php?t=360684

"On my Kindle (Paperwhite, 11th gen), the dictionaries are held in the “\documents\dictionaries” subfolder (I kept my firmware to an older version to keep my USB connection). When I bought them online (on Amazon, see Kindle Default Dictionaries category), I received pre-made MOBI files that I only had to place in the aforementioned subfolder, without converting them to other file formats. Afterwards, I set up my default dictionaries for every language on my Kindle in Settings → Language & Dictionaries → Dictionaries. Hope that helped. "