Without it, it did not load the translation keys on my local installation as it was ignored by the plugin manager.
This is because we check here for a valid plugin structure https://github.com/piwik/piwik/blob/3.0.4/core/Plugin/Manager.php#L301 but Contents has neither plugin.json nor a plugin file
There is a plugin file: https://github.com/piwik/piwik/blob/3.x-dev/plugins/Contents/Contents.php
true, sorry about that. I must have deleted it accidentally in my branch