The Text File simply stores the text available for the game. Any field which the game attempts to display is first looked up here, and if it matches letter for letter with what is contained between any <tag> and </tag> markers, the appropriate language text will be displayed instead.
5 Languages are supported by the basic Civ engine, English, German, French, Spanish and Italian.
Some special Icons are available for use in the Text, as well as numerous Formatting Commands. These are all contained within standard brackets [ ]. You can also display text which has been passed along with the request for information display.
For Mod-Mods it is possible to make your own Text File so as to avoid including all of the FfH text with your mod-mod download. Just name it CIV4GameText_*, where the * is replaced by any text of your choice. Be warned that if a field is duplicated then the last one loaded will be the text utilized in-game (load order appears to be Alphabetical)