I added “El Hobbit” in Spanish as a new book request by searching for it in the Spanish section of the website, then when it wasn’t present uploading a link to Amazon ES URL. (Safari, mobile)
I was asked to estimate the level for the book so I opened another tab, went to my Japanese account, and looked at ratings for children’s books I’d read in Japanese.
I then completed the request for El Hobbit on the original tab. The book was added to my Japanese account, rather than my Spanish account.
Oofta. Very nice catch! Well, I fixed El Hobbit to be Spanish and I’ve updated all the corresponding items (your review, your library… etc). If there’s anything that doesn’t look right, let me know… although hopefully my alerts in this regime will alert me too if there’s errors.
Anyway, I went ahead and fixed the upload process to determine language for the book based on the url your submitting (amazon.es → spanish) rather than your current language setting.
That should prevent this bug for the future, so i’ll close!
Also, welcome to Natively Spanish!! 
