www.makemkv.com

Which converter (MKV -> mp4) would you advice me to use, if I want to find to keep all the subtitle tracks that makemkv was able to keep when I rip a Blu Ray disk?

Thank you.

You cannot keep PGS subtitles from a BD rip if you use MP4 as your video container. They are not compatible, and you can only burn one subtitle into the video.

At the very least, you would have to convert the subtitles into a format that is MP4-compatible, and that is not an exact science.

Another option... rather than conversion, some folk have found it easier to download .srt files from sources like OpenSubtitlesdotorg. Plain text, generally supported by players. Just pick from a blu-ray source & download what others have already converted. YMMV