Handbrake 1.51.1 (current installs beautify I think) and opens, but instead of any characters I can read, it has a Boxes where characters should be. I assume that it is stupidly trying to display a font that it doesn’t have. I’d give it the font it wants if: a) I had a clue what it was, b) I knew how to provide it. I’m assuming that this isnt’t Handbrake.fr issue, but is a flatpak issue. Thanks to anyone with a clue.
Did you try to run it from the terminal via flatpak run fr.handbrake.ghb and see if it logs something?
It might also be interesting, which language you would expect it to use (might be a different font) and you should probably still report it to the handbreak people, as they seem to be manging the flatpak. See GitHub - flathub/fr.handbrake.ghb