You should install CB Plugins (including CB Language plugins) using the CB Plugin Manager installer and not the Joomla installer (used to install Joomla extensions, modules, plugins, templates and language packages).
If you try and install a CB Plugin using the Joomla installer you will see a message similar to:
"JInstaller: :Install: Cannot find Joomla XML setup file"
This message is generated by Joomla and it tells you that the package you are trying to install via the Joomla installer does not have a Joomla XML file in it. Of course it doesn't, as it is not a Joomla package. It is a CB Plugin package that needs to be installed using the CB Plugin Manager.
On the other hand if you try to install a Joomla module (e.g., the CB Login module) using the CB Manager installer you will get a message similar to:
"ERROR: Could not find a CB XML setup file in the package"
This message is generated by CB and it tells you that the package you are trying to install is not a CB Plugin package. This will happen if you try and install a Joomla module (e.g., the CB Login module) or a Joomla System or a Joomla Search (e.g. CB Search) or a Joomla Content Plugin (e.g. CB Content) using the CB Manager installer instead of the Joomla extension installer.
Please use the Joomla extensions installer instead. Typically, Modules, Components, Joomla Templates, Joomla Languages, Joomla system and content plugins should not be installed using the CB Plugins installer.