I checked, and it does seem to be necesary to change something other than just expor_descr_buildings.txt

I played as germans, after giving myself a city and 12050 population to start with, and I could not build past the 'minor city' level'.

I think that you should find the settlement plans (they are in the packs, I believe), and copy one of the other culture type's plans for large_city and huge_city into the appropriate folder, and rename them with BARBARIAN instead of whatever the previous culture was. Then I would expect that you will be able to have large and huge cities as barbarian factions.