Translate Flexible Invoices with Loco Translate plugin
Loco Translate is a plugin to manage your translation directly from the WordPress dashboard. It is entirely free, so all you have to do is install it on your site. First, navigate to your Plugins. Then click the Add new button. And find Loco Translate on the bottom left of your Dashboard. Finally, install and activate the plugin.
First, choose the plugin, list all active plugins, and click Flexible Invoices for WooCommerce. Secondly, click New Language and select your language from a dropdown. Please remember that you also have to choose this language from Settings > General to show translation on your site.
Then select the location of your translation and start translating the plugin. Now, click on every string one by one and translate it to the language of your choice. Save your work and check changes on your site.