Too bad you're using 2.72, otherwise you could run Game.ChangeTranslation in the game_start for each game. Not a very helpful response, sorry. My guess is yes, it would work to have one huge file, and yes, it would slow the game down (because AGS searches through all Strings in the translation file until it finds the right one). If I remember correctly, your game uses its own internal storage of multiple Strings in a single line - and this may in fact speed it up a bit, because you have less Strings to run through. But for actual performance there doesn't seem to be any way to find out short of testing it.
Edit: Also, doesn't your cross-game code need to know the translations for all the games at all times? If you changed translation files between games, any words in the history list that didn't have a translation in the current game would appear in English.
Edit: Also, doesn't your cross-game code need to know the translations for all the games at all times? If you changed translation files between games, any words in the history list that didn't have a translation in the current game would appear in English.