Thanks for reaching out about the translation support for the plugin.
The plugin uses the www.ads-software.com translation platform workflow to load and generate the translations in the back-end admin and front-end.
All translations to render correctly in the back-end admin need to be checked into the French repo for the plugin: https://translate.www.ads-software.com/locale/fr/default/wp-plugins/product-add-ons-woocommerce/
Once the translations are 95% complete for the language under the Stable strings, when the WordPress General language settings are set to French, the translations for the plugin will automatically import and display for both the front-end and back-end admin strings.
For example, Slovak has provided translations for the plugin, now when the language is set under the General settings, the translations are displaying in the back-end admin from the www.ads-software.com translation platform database, as shown as reference: https://ibb.co/NWdvs6g
We’d love for you to contribute the French translations to the www.ads-software.com translation platform allowing other community members to enjoy the plugin in their native French language. The French polyglots team, https://make.www.ads-software.com/polyglots/teams/?locale=fr_FR can also help support the translation and approval of the translations to make them available for the plugin.
We hope this helps clarify the translation process for the plugin and we look forward to seeing the French language translated for the community. Thank you for the support.