Open up the assets1.pk3 and look in the "strip" folder. Extract the objectives.sp file into your project folder.
Now edit the objectives.sp file to change the text for one of the objectives. For example, the first one looks like this:
TEXT_LANGUAGE1 "Investigate the abandoned Imperial outpost."
TEXT_LANGUAGE2 "Fouiller l'avant-poste impérial."
TEXT_LANGUAGE3 "Verlassenen imperialen Außenposten untersuchen."
Just change "Investigate the abandoned Imperial outpost" to "The quick brown fox jumps over the lazy dog" or whatever.
Important: You can't add new objectives to the file, nor can you change the REFERENCE name of the existing objective. In the example above, it has to be KEJIM_POST_OBJ1, and that's how you refer to it in your ICARUS scripts. I've tried and tried to find a way to add or rename objectives, and if anyone has found a way let me know because I sure can't do it.
Now save your objectives.sp file and put it in a folder called strip in the pk3 file for your map. When your map loads, the game will prefer the objectives file in your pk3 file.
Now you can use your newly edited objective in your scripts. Set it up like this:
set ( "SET_OBJECTIVE_SHOW", "KEJIM_POST_OBJ1" );
When the player achieves an objective, use SET_OBJECTIVE_SUCCEEDED. Voila!