You should be able to configure portal to only provide search results in only the language(s) you desire.
The option for that is in portaladmin, and started at 10.3.1.
There's not yet a way to remove the default content that ships with Portal for ArcGIS, however there is an existing enhancement request for this functionality.
ENH-000090864 : Cannot remove pre-existing content from portal for ArcGIS