If you want to change some of the predefined texts in the user interface in any properly coded WordPress plugin you must translate it.
Many users are confused and think plugin translation is useful only if you want to use the plugin in another language.
Translation is what you need to do even if you only want to change some words. It’s translation from English to English – there is no difference. So please check the link above, translation a plugin is not scary at all.
In case your WordPress installation is default, your plugin language files need to end in en_US.po and en_US.mo. Not en_EN please, there is no such locale.