As stated Community Builder has language plugins (see CB Plugin Types) that can be used to localize CB in any language. CB also has a set of default language files (see CB API document) that are pre-installed during CB installation. These default files should not be altered as they will be over-written when you upgrade to a new CB version.
The default language files contain default strings in English. This means that if your Joomla site is set to a "French" default language and you have not installed the relevant CB French language plugin package, then since CB cannot find a relevant French CB language plugin, it will display the default strings (which as stated are in English).
If your website is only using a single English language and you are satisfied with the default file wordings used in CB, then you can leave things as they are and there is no need to install the CB English Language plugin.
If however, you are not satisfied with the default texts, then you should not rush to find and modify the default language files, because these changes will be lost when you upgrade CB in the future. Instead you should download the relevant CB English Language plugin, unzip it locally and modify the relevant strings as you please. Then repackage (zip up files again with changes) and install this modified to your liking CB English language plugin as a CB Plugin (using CB Plugin installer).