The books are only worth what buyers are willing to pay for them. Would you pay for a book face value if an owner already had it for a few years, even if it’s in good condition? Something that’s previously owned automatically depreciates in value so you ought to price it in such a way that the buyer feels like they’re getting a deal.

You owe it to yourself to learn the language. Since you read so much, pick up a book in that language. I’m sure you could find a good suggestion for beginners.