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.
Loco plugin is the best solution to rewrite the strings. You need to create the translation in the same language as your site is, and just re-write. You can delete the plugin and this change would remain.
Loco plugin is used not only for the translation but for the rewriting as well, you need to create the translation in the same language that is set in the site and all the strings would be changed.