I finally found a solution to translate:
In the file /new-user-approve/localization/new-user-approve-fr_FR.po, I replaced the line:
#: includes/messages.php:87
#, fuzzy
msgid "{username} ({user_email}) has requested a username at {sitename}"
msgstr "%1$s (%2$s) has requested account approval on %3$s"
With :
#: includes/messages.php:87
#, fuzzy
msgid "{username} ({user_email}) has requested a username at {sitename}"
msgstr "{username} ({user_email}) has requested account approval on {sitename}"
Then I replaced the new-user-approve-fr_FR.mo file with a new one generated using an online converter (Example: https://po2mo.net).
Now the email is translated, and the variables work.
Best regards
