Hey Lexx, i got the same problem.
I wanted to make some little fixes on Fallout 3's french dialogues.
So far, i could locate the Dialog lines with
fomm and TESsnip (thanks a lot Timeslip), but i couldn't understand what is shown and how can we modify the dialog lines.
=> Dialogue lines seems to be in Fallout 3.esm\GRUP (DIAL).
EDIT : I've managed to make it work, but i haven't tested it in-game yet.
OK, when you have opened GRUP (DIAL), you see all the dialog trees listed : Double-Click on "GRUP" just after the name of the Dialog Tree (like DIAL (PLAYERLOCKEDOBJECT) and you will see some new entries "INFO"
Each "INFO" is a dialog line, when you click on one "INFO" check on the bottom left of the screen in the "Name" column, you will see DATA, QSTI, TRDT, NAM1, etc...
Double click on NAM1 and you can edit the dialog line !
Now, maybe someone could help us to make a new .esp file which will only contain the modified dialog lines instead of editing the whole Fallout3.esm