Language manager enables admin to edit language files , please check in order to perform you should write rights on language files. Editing files could be risky if you don't know php syntax.
Once you double click on file it would open in editor

This extension is vqmod based so no core changes. Just drag and drop , it works fines with default opencart admin.
Extension is made by the freelancer who holds opencart reviews on freelancer.com
If you have any questions you can mail me at shambhalnetworks@gmail.com