Yes this "ACP_FEED_SETTINGS" is only present in that file amount all language files in phpbb. You can either open the file and search for it, if not there add it. But my guess is if that is missing other stuff is as well.
You can only replace it with a new fresh file if you are absolutely sure that you have not added any extra stuff to that file via mods etc. If its a fresh board, yes you can simple overwrite it.
If its en or another language is up to you, whatever is standard for your site. Just dont change the name of the directory because it looks smarter
Language file wise "en" is default not en-us