[ext] bug: translation of the action buttons under YouTube videos not set up

[maximedrouhin]

The following buttons are not only not translated to French, but it also seems like translation for the relevant strings is not set up (see CONTRIBUTING.md#translation).

Here's the Discord conversation from which this stems:
https://discord.com/channels/760540475139817472/760540475668824118/1206003608240529418.

Warning: At the time of writing, there's a pending PR (#1916) that changes addVideoButtons.js, which is where the buttons are named with hard-coded strings. It might be best to accept it before taking care of this issue.