Hello Riatre,
thank you very much for your generosity by sharing the tools and source code! This is certainly a very helpful contribution for the community's translators, for those who like messing around with games, and for those who enjoy the game's music.
For anyone having problems, here are some rough instructions on how to use these tools to translate the game:
The first thing to do is run "th135arc.exe /x th135.pak". This will extract all files from the th135.pak file into a subfolder called "th135".
Some of the extracted files will still be compressed with zlib, the TFBM format for bitmap images, and the TFCS format for .csv files (containing in-game text).
Run "extractBM.bat" and "extractCS.bat" to automatically decompress all supported files in the subdirectories into an easily editable PNG format. This process may take a while for the image files, and it will not extract the 8-bit-paletted character sprites (which are irrelevant for translation anyway). Also, some files may be skipped due to "invalid filename", this is because of locale problems with the file names and should be fixed by AppLocale (can't test this since I am on Wine at the moment).
Translating the image files should be fairly trivial, you'll find most text images in the "System" folder.
The .csv files are text files in Shift-JIS and not only allow you to translate names and descriptions, but often also control game functionality, allowing you to change skill/spell card script functions, mana cost and faith values, as described by the first line of each file. I'm tempted to just mess around and mix characters' cards up a bit, but I don't want to risk corrupting my th135.pak at the moment, and this is pretty much unrelated to translating the game anyway.
The .nut and .act files can be decompressed using "dumpcnut.bat" and "dumpact.bat", respectively, and seem to contain strings related to Squirrel scripts. These should probably be left alone for now.
Before continuing, make a backup of your th135.pak and th135.exe. This step may corrupt the file in some way if you haven't set up your locales properly.
To repack the archive, run "repackBM.bat" and "repackCS.bat" (and optionally "writecnut.bat" and "writeact.bat"), then run "th135arc.exe /p th135", followed by "th135arc.exe /h th135.exe" to change the security hashes in th135.exe and allow the changed th135.pak to work. This worked partially for me: the game still runs and displays most images, but stage backgrounds do not work anymore. To revert to the original version of the game, restore the backups of both th135.pak and th135.exe. Restoring only the th135.pak file and then running "th135arc.exe /h th135.exe" again does not seem to work at the moment.