Well, GUI changing aside (which is a limit), there are two ways to go about this for the ingame/level stuff.
One is by using the Divinity Toolkit. Create a new mod, make it dependency on main and use main data pack. You would load up the dialogue menu, copy to 'your mod' then alter the text. I have seen a video stream where this is shown, and I believe this is covered in the new youtube videos.
http://www.youtube.com/watch?v=Mc2Mzk5Z6LYAnother way would be using lstools. Using the module made via Divinity Engine, you would unpak the main folder, and copy over the localisation files into your mod area. Then using that program, you edit the .lsb files directly. You would also need the Subtitles, which can be edited by notepad++ easy enough, same with the raw files for the Story Editing. You would need to need to 'build story' (under Story Editor) using Divinity toolkit once you made those changes.
Method 1 I believe is the Larian approved method which should be more reliable, but method 2 would be brute-force method which should work as well.
Edit: By the looks of it, you would need to do the second method for a lot of it, such as the subtitles. Anyway, best of luck!