Hello Maxime, I’m not sure we are talking about the same widgets. The strings are not registered in the plugin, it’s an iframe loaded by your script. Translating the plugin would have no effect on this if I understand correctly.
I have seen these components translated in french on another website using Sezzle but built with Magento.
iframe : https://i.postimg.cc/TPtmycmG/Screen-Shot-2022-03-25-at-9-52-37-AM.png

Product widget :https://i.postimg.cc/50Z3PM3n/Screen-Shot-2022-03-25-at-9-52-32-AM.png
