[mob
ius]All you need to do is editing the messages.tbl table. I'm attaching the retail one to this post so that it's easier for you to get it. It can be easily extracted from the root VP in your FreeSpace 2 directory by using a dedicated program, like VP View.

Open the table with Notepad and make the changes you want to make. Please note that, if you alter the text, it's appropriate to change the numbers you see at the end of every entry and replace them with "-1" (without the quotes). Doing that prevents people who changed language via the registry (and switched to, let's say, German or French) to get retail texts instead of the ones you created.
Finally, it's worth noting that you need to install the new table as a mod - check Karajorma's FreeSpace FAQ (you can find a link in my signature) and look for the installing mods tutorial. The modified table will go in <YourModFolder>/data/tables. Note how you need to create subfolders if you want your mod to work properly.

[/mobius]
[attachment deleted by evil Tolwyn]