Hello, yes, you are right.
What you are looking for is really in systemmsg-e.dat.
You can open this file with L2FileEdit.
Download L2FileEdit: download
Select chronicles "Inter;ude".
Click "Open & Decrypt" and open systemmsg-e.dat
Then you can edit text into editor, or try to copy all to notepad, after changing try to paste to L2FileEdit.
Next click "Save & Encrypt". But be careful, use 413 encoding.