I know that many know this already,but i just thought to put it in here for the new gamers to see.
To unlock Celt and Berber factions in Barbarian Invasion.Go to Bi/data/text folder and in there campaign_descriptions.txt file, scroll down the list and when you see the Celt and Berber factions,just chance the text from NON-PLAYABLE to MODERATE.
Save and move into folder Bi/data/world/maps/campaign/barbarian_invasion and in there to descr_strat.text file remove celts and berbers from the list of the unplayable factions and ad them to the list of the playable factions.After that save and open the game.Now Celts and Berbers are available as playable factions.Enjoy!