Oh shoot. I think you're right, I must have mixed in the Italian language file at some point. I just ran a full new re-translation, and although I don't speak neither Italian nor Spanish, I can see that some strings that looked Italian now looks more Spanish. But I have no idea if it's correct.
I'm using a system called Weblate. If you would take a look, I'd be very grateful! It's a bit messy on the first registration, so make sure you re-visit this exact link after registering a user:
https://weblate.erlenddahl.no/projects/jetreader/
(I haven't actually tested it with external users yet, so please let me know if you hit any problems.)
Note that the strings in your app may not correspond fully to the strings in Weblate, since I ran a full re-translation, and I've also added some new strings while working on a new update. Might be easier for you to wait for the update, and then if/when you spot anything weird, you can go into Weblate and search for the weird strings.