Don't modify core code. If you do then upgrading is going to be painful for you every single time. CB has a lot of triggers and in 99% of the cases what you want can be done by acting on those triggers. For example CB Auto Actions is a tool specifically designed for just that.
As for language files you need to install a language plugin and modify it. For example install the English language plugin then make your changes there. See the below for language usage information.
www.joomlapolis.com/cb-solutions/languages
For modules we support Joomla layouts so you could supply a layout override to your Joomla template so upgrading won't cause you to lose any layout changes you may have made.