TF2/Modifying the Mission Briefing
The mission briefing, shown to players before they enter the match, is set automatically for Basic Capture the Flag and Control Point games, just by putting the right goal entities on the map.
Custom mission briefings can be done, but how?
One way is to use localized texts:
In the directory where your .bsp is (_mapname_.bsp - Usually "..\Program Files\Valve\Steam\SteamApps\_Username_\Team Fortress 2\tf\maps\"), create - or extract with GCFScape and rename for your map - the localised map .txt briefing files:
mapname.bsp mapname_danish.txt mapname_dutch.txt mapname_english.txt mapname_finnish.txt mapname_french.txt mapname_german.txt mapname_italian.txt mapname_japanese.txt mapname_korian.txt mapname_norwegian.txt mapname_polish.txt mapname_portuguese.txt mapname_russian.txt mapname_schinese.txt mapname_swedish.txt mapname_tchinese.txt
In each of those, is the briefing text displayed for your map, in the named language.
(Please do edit this to be more clear)