Language loading is entirely done by Joomla then by core CB. Whatever language Joomla has loaded is checked in CB. CB then tries to find a CB language plugin based off the language tag. If one exists it will be loaded and if one does not it will fall back to default language. If another extension for example changes the loaded language CB will see the newly set language. Some plugins will also do a language change, but should all be reverting it back, when sending notifications so notifications are in the recipients language. CB Activity has no language changes so it would be a loading issue outside of CB Activity.