There's a few files you'd edit, each supply the text for a different part of the game.
esf\titles.txt has a lot of information you'd edit, just make sure you edit it right:
titles.txt said:
//team menu
Title_SelectYourTeam //don't touch this identifier
{
Choose your side //this is what you change, it's the text that appears ingame
}
All of the entries in that file are stored the same, just edit what's in the brackets.
esf\commandmenu.txt is, well, the command menu. It's used for the bots. Edit what's in the double quotes.
The character descriptions (what shows up on the "choose your class" VGUI when you mouse-over each character) are stored in
esf\classes. Each character's information is in a separate file, editing those are pretty self-explanatory.
There's probably some other files, but that's all I see right now.
titles.txt is the big one.