Finally: first version of a tool that lets you edit the scripts. As always, you can find the release here. It is designed to doing a little bit more than just editing dialog texts in the future (maybe...), but for now the thing you want to do is:
Open the .scn file you want to edit (they contain the texts and are stored under dat/<your-language>/ui/scn/)
Click on "Dialogs" > "Export..." to export them into a JSON file.
Edit them to you desire
Open the same .scn file in the SceneEditor again (if you closed it meanwhile)
Click "Dialogs" > "Import" and select your edited JSON file
Click "File" > "Save" (or "Save as")
In theory, you could also edit the texts directly in the program (I recommend to enable "View" > "Hide Unknown Instructions" for this), but I guess having everything in one text file might be a little bit more comfortable.
When you edit the JSONs you should remember to escape your quotation marks (") with a backslash (\") so that they don't destroy the JSON document. And of course: make backups of everything before overwriting the game files, stuff might not work correctly
If you encounter any problems or find dialog parts that are not present in the exported files (some of them might be using different instructions that I didn't see yet), just tell me and I will try to fix it.
Also, tell me if the JSON format is too uncomfortable to edit. It was just the easiest to export and import for me, so I chose this format.