Internationalization of a plugin doesn’t mean that you can change its language by setting some option.
It means, that translation for this plugin is provided within the package, so after changing the language of WP, the plugin will use the same language as WP does.
If you want to change the language of WP, you can go to Settings -> General and there is a “Site Language” select:
After switching it to French, you can check, if your plugin is correctly translated – your translations should be visible from now on.
And if they’re not, then you have to check, if you’ve translated that plugin correctly. To do so, you’ll have to:
- Define textdomain in your plugin.
- Use that textdomain and translation functions for all your strings in the code.
- Provide correct translation files (.po/.mo).
- Make sure the paths to these files are correct.
And it’s also worth to remember, that for plugins hosted in official Plugin Repository, you can use translate.wordpress.org platform. More on this here:
https://make.wordpress.org/plugins/2015/09/01/plugin-translations-on-wordpress-org/