Why don't you use po/mo for translation?
If we use po/mo for translation, this plugin will be displayed in the language you determined at the WordPress Admin Panel.
We don't use the po/mo system in our plugin in order to automatically display the most optimal one by the language setting of the browser of visitors.
So, you have to edit a conf file of this plugin in order to translate this plugin. We are sorry for the inconvenience.
( 2019/August/17, 11:56:28, JST )