These issues are not actually the plugin, but a result of incomplete translations.
The official plugin translations are all handled through WP’s polyglots project (see: https://translate.www.ads-software.com/projects/wp-plugins/wp-members).
The language packs that are included in the plugin download are only there for legacy purposes (such as when there is now complete language pack available on wp.org) and they are no longer maintained.
In your case, French is listed as only 40% complete on polyglots. Language packs only download with the plugin when they are 90%.
Of the specific issues you mentioned, the TOS text is the result of a change in the string for that particular field. It was “Please indicate that you agree to the TOS” and “TOS” was changed to “Terms of Service”. So in this instance, there is no current match for that in the language pack.
There are a number of solutions. First, it would be great to have a complete French translation – including that particular string. That’s not going to happen until people maintain the language pack on polyglots. One simple solution is that the field label can be customized. That’s true for any field, and while translating via a custom field label isn’t an actual solution for a missing translation string, that does solve the problem. Lastly, that particular field’s label text can be filtered directly with wpmem_tos_link_txt.
The other issue – the register button – is the result of the translation being marked “fuzzy” in the translation file. Any translation that is marked “fuzzy” in the .po file will not be included in the resulting compiled .mo file. Similar to the other translation, the ideal solution is a proper translation file. But since that doesn’t exist yet, that element can be filtered in the wpmem_register_form_args.
Also, some people use the Text Editor premium extension for customizing these and other strings. It allows an easy interface for customizing all user facing text for the plugin.
There is more information on plugin translation, including information on becoming a translation contributor, at this FAQ:
https://rocketgeek.com/plugins/wp-members/docs/faqs/#translation