The checkout button is the Woocommerce functionality, you need to create the translation in Woocommerce as I mentioned above:
Please check the video tutorial to see how to save the translation file correctly. You can remove the plugin and translations would remain. Also, translations would remain after each theme update.
In order to translate the WooCommerce strings, you need to create a separate translation in Loco > Plugins > WooCommerce in the same way as shown in the video.
If you have any questions please feel free to contact us.