Some of it is in expanded.txt or expanded_bi.txt (use the latter if it exists in your data/text folder)
{LOMBARDI_DESCR} Lombardi\nGood mix of barbarian troop types, plus heroic warriors!
the other bits are in data/text/campaign_descriptions.txt eg:
{BARBARIAN_INVASION_TITLE}BARBARIAN INVASION
{BARBARIAN_INVASION_ALEMANNI_TITLE}The Alemanni Tribes
{BARBARIAN_INVASION_ALEMANNI_DESCR}Difficulty Factor: ESTIMATED MODERATE\n\nThe forests of Germania have.....
Was that what you meant?