www.makemkv.com

Hi, I have an original foreign Blu-ray that isn’t available in my country. I was wondering if it’s possible to rip the whole disc and then add subtitles in my language from the DVD….essentially replacing the French subtitles. I don’t mind if selecting French in the menu brings up my language instead, but is this possible?
(I know how to add subtitles and sync it with the video but that is without any menus)

You should ask Google: convert DVD subtitles for BD

Depending on your player, you can copy DVD "vobsub" subtitles in as track on a Bluray MKV file, and they will play just fine.

MKVToolnix is the utility to do this with. I used to do it when my TV wouldn't play the PGS subtitles found in Bluray disks, but haven't needed it since I upgraded my TVs.