This is for Community Builder 2.x
First, download the appropriate language plugin and unzip it.
Edit language.php for front end strings and/or admin_language.php for back end strings.
Re-package the folder (zip it up).
Install it via the Community Builder Plugin Installer.
I wrote this because the instructions for doing this are very lengthy as they're directed at people that want to edit the strings in a language other than English.
By the way... don't change the name of the file, etc. I changed the file name so it was, more appropriately, EN-US (instead of GB). And changed other associated strings in the XML file. It doesn't work. You have to leave it GB.