I'm confused now, too.

You would change that (Categories) in the same file where you changed the Manufacturers wording (english.php).

You say that when you do that, the whole page disappears. One thing that will do that is special characters (like an apostrophe or quotation mark) in a define, so if you tried to change 'Categories' to 'Bob's Categories' you'd get a blank page. It doesn't sound like you're doing that, but using the CPanel editor may be causing the problem.

This would be a good time to learn about FTP - it'll save you a lot of trouble and make styling your site a lot less stressful.

Get an FTP program - many here use Filezilla, I use the FireFTP plugin for the Firefox browser. Then enter your server login details, and you'll end up with a screen with two Windows Explorer-type panes - one showing your local computer's files, and one the server files.

Find the english.php file, drag it to somewhere on your local computer, open it in a text editor (like Notepad), make your changes, save it, upload back to the server.

Once you get the idea of it, it's second nature.