Editing button text on Appointments+

I'm trying to edit the text on the payment confirmation button in Appointments+

I received advice via the Live Support channel and have added language files (.mo and .po files) to my site. But, when I edit the wp-config file to set the language to en_GB and save it back via FTP, the site does not load. Instead a Parse Error is shown. I've tried using Notepad, NetBeans and WordPad to edit the wp-config file. I can see no errors in it. Please help!

This is a showstopper issue for this website, as the payment confirmation button needs to clearly show that it's for a deposit only.