I have found the problem!

I checked the contents of BlendSwap-CC-0.sh3f again and noticed that there is PluginFurnitureCatalog.properties but also PluginFurnitureCatalog_en_US.properties. In the en_US file, the item names + categories still had the original names and did not contain my modifications.

My Windows was set to US English, after I changed it to UK English everything was exactly where it needed to be in SH3D [:D]

Then I thought I'll try to remove the PluginFurnitureCatalog_en_US.properties file from BlendSwap-CC-0.sh3f and changed the Windows language back to US English and it worked as well. Problem resolved [:)]

Thank you so much harbinger, I would have never thought to look into the contents of the *.sh3f file!