Editing the text files and images shouldn't a problem, but being unable to extract and repack the DATs is what makes it hard.
I wanted to translate Seihou games for the longest time, and here's what I know about the games' DAT files:
I don't know much about Shuusou Gyoku, except that you can extract game data from DAT files (PBG format) with Brightmoon.
Kioh Gyoku uses pretty much the same DAT format as EoSD (PBG3), but there's a catch - unlike Touhou games, where each DAT file contains only files and no folders in it, Kioh Gyoku's DAT files have folders with files inside them, and thdat doesn't recognize those folders (it's also not possible to add folders when creating a DAT file).
I did get in touch with VBChunguk, who updated Touhou Toolkit to version 7. He told me that thdat is made to consider folders only for SWR and Soku; the Folder extracting/archiving feature is already there (it is just "hidden"), and he could add support for Kioh Gyoku with a little touch (and, even if he'll add support for Seihou, that version of Touhou Toolkit won't be the main branch, because it's "Touhou" Toolkit), but nothing came out of it yet. I sent him another message on Youtube a week ago, but he hasn't replied yet. You can try contacting him on
Twitter, he seems to be quite active there.
Nmlgc did make a PBG6 Data Extractor for Banshiryuu's C74 version (you can find it at
THwiki's "Tools and Modifications" page, complete with source code), but there is no way to repack the extracted files back.
C67 version of Banshiryuu is like a completely different game, and is also worth having a translation patch (it also has like ten times more dialog!). It uses PBG5 format for DAT files.
Edit:
I
found a handy little program called
ACXDecipher that can extract game data from .AC5 (C67 Banshiryuu and Samidare) and .AC6 (C74 Banshiryuu) files. However, unfortunately, it doesn't have the repack function.