We decrypted the rest of the message with the variation you used and translated it from French:

Like my idol Olivier Levasseur, La Buse, I have hidden my treasure and left behind clues as to its location. Your next hint is in the town that this bottle was manufactured.

This guy seems like a bit of a nutjob, but I'll gladly take his treasure! Can you help us find where this bottle was made? We got a little... excited when opening it, but I'm sure these small bumps at the bottom will be enough! They were hard to see in a photo, so we transcribed them on a piece of paper.

Enter your flag in the form: BCCTF{Location}