I'm not entirely sure I understand your post.
There are two different language options in Gospel Library for Windows. The first is the Content Language--this allows you to access a whole catalog of gospel content in a given language. The second is the User Interface Language. This allows you to use the app in the language, no matter what content language you are using. Each has its own location (or locations, in the case of content language) in the app.
To change to content language, simply click or tap on the language dropdown at the top-right of the screen. To change to user interface language, tap the settings button in the navigation bar on the left side of the screen, then chose the language you want to use for the application's interface under "Languages". Changing the app's interface requires you to close and reopen the application so that it can reload the user interface in the new language. Changing the content language does not--it will just load in the new selection of books available in the selected language.