The default English language files are just packaged with the main CBSubs plugin like our other plugins. It doesn't have or need a separate language plugin for the English strings anymore. This used to be required as our old defines usage didn't have a fallback usage, but with CBTxt usage we don't need to force you to use a English language plugin anymore. To translate CBSubs you'd do the same for it as any other plugin and place its language files in the plugins folder of your CB language plugin. See the CB 2.0 Primer Book and the below for further details.
www.joomlapolis.com/cb-solutions/languages