When you change zone ingame, appears a message to you: "Zone Name - %s Territory". %s is the faction (Horde or Alliance).
I tried in FactionGroups.dbc, but when I change Horde to Horda (example), the game bugs and I cannot create new characters.
Tried in GlobalStrings.lua and GlueStrings.lua, but no success too.
Someone knows another file that contains strings to translate?