Suggestion.
Gauls:
Ambigatos: Bonus in population capacity given by any CC
Vae Victis: Resources acquired from destroying and capturing buildings
Headhunting: Resources acquired from killing heroes (or champions too).
Invented the Reaper: Bonus in collecting food from farms
Montefortino: team bonus, reduced blacksmith tech cost and research time
Britons:
Sheep Age: Bonus related to livestock
Inis Mona: Druids increase attack speed of nearby units (or reduce attack speed of nearby enemies)
Build, Endure and Rebuild: Construction bonus (reduced time or reduced cost)
Britanni vitro inficiunt: Bodypainting gives infantry bonus (speed, healtpoints, health regen?)
Masters of Earthwork: Reduced stone cost for CC and walls.
Teaching Center: team bonus, reduced resources cost for healers.
Celtic Epic / Mabinogion: team bonus, reduced training time for champion (or heroes)