I dont think there are any default language files... this may be a good idea to implement though. Do you just want to change the "xxx joined the game" message? If you do - just give me the translation and i'll make a quick plugin for you.
Thanks for your interest, i managed to find a solution, though it took me a little time. I installed NetBeans, downloaded the CB source, changed the texts and compiled
If you just want to change ''xxx joined the game'' you can use the HeroChat plugin. For other messages i recommend HeroicDeath and MessageChangerLite.