- πΊπΈUnited States illeace
As of 2.9-alpha6 and Drupal 11, there is a robust set of French translations for H5P, including most of the strings in the files you mentioned above. Further, I've confirmed that strings like the "description" value in files like h5p.links.menu.yml can be translated via .po files and with custom string translation in the Drupal UI at /admin/config/regional/translate.
I realized that this does not at all answer the actual questions asked in the post. For the benefit of future visitors to this issue, this page provides an overview of Drupal's localization process β with guidance on how to contribute.