So is everything OK now?
I think from what you have said so far you have created a UI-hook that changes the navigation portlets view.jsp so you have a different view (although most changes to navigation are done with styles as the main view is pretty basic looping around the site structure providing nested pages.).
You asked about Language_en.properties that is the main English language file (Language.properties is the US English default language file.)
You don't edit it so you don't need to know its location
You use a hook to either overwrite specific keys or to add your own keys for text strings you have added to your jsp files.
The same hook as the UI changes can be used - it is easiest to use the IDE , right click on your Hook project select NEW > Liferay Hook Configuration > select he last option - Language Properties and follow the wizard.
Overwriting a value will changes its display everywhere that key is used, so sometimes you need to change the jsp file and change the key to your own if you think you need to only change one instance of the text string.
All of this is done in a hook.
