Was looking into this. I looked a long time ago and didn't get too far, but I think I did better on it this time.
.NPK.ZZZ are compressed with zlib.
The part Stormyu didn't say: The SYSTEM.VOL is data XOR'd with 0x55. It appears to be uncompressed.
All the text is UTF-8.
It looks not too hard but I've said that before and been wrong, like for the ZZT3 project.
The first step will be to unpack SYSTEM.VOL into its component parts and de-xor them, and also make a re-packer.
I noticed a huge number of dummy entries in the items table, but I think all the real entries are there.
The XOR thing - that's just an obfuscation technique, right? I worked on Wander Wonder (PC) before and that had all the files XOR'd with 0xFF (all bits flipped).
Shot of Quest screen:
Shot of Place Names in-game:
If you want to see what I'm working on:
https://docs.google.com/spreadsheets/d/1H_noLOY4p2d3VgoZsVluCMWEKF31C2udZS-dtD2Lfso/edit?usp=sharing
List of stuff:
menu_resident.npk (sub-file under SYSTEM.VOL) - main menu
Z0_0 - Style menu (these do not have a filename I could find, sub-"files" under Z0.npk)
Z0_1 - Item menu
Z0_2 - Equipment menu
Z0_3 - Skills menu
Z0_4 - Trial Log menu
Z0_5_0 - Ranker File menu
Z0_5_1 - Ranker File descriptions
Z0_6 - Options menu
MENU_TRIAL_LOG.NPK.ZZZ - Trial Log data for when you check training boards. 41 missions in the game
DB00 - System messages (sub-files of DB_resident.npk which is a sub-file under SYSTEM.VOL)
DB01 - Ranker's names
DB02 - Swords
DB03 - Sub-weapons
DB04 - Accessories
DB05 - Armor
DB06 - Misc item (recovery, etc...)
DB07 - Key item
DB08 - Blank
DB09 - Sword descriptions
DB10 - Sub-weapon descriptions
DB11 - Accessory descriptions
DB12 - Armor descriptions
DB13 - Misc item descriptions
DB14 - Key item descriptions
DB15 - Blank
DB16 - Ranker's titles
DB17 - Ranker's titles (Japanese)
DB18 - Ranker's names (again)
DB19 - Mission titles, probably for the mission clear message
DB20 - Style names
fld_resident_1 - Field map place names (sub-"files" of fld_resident.npk which is a sub-file under SYSTEM.VOL)
fld_resident_2 - Action names
fld_resident_3 - Names for world map
IF_TRAVELMAP.NPK - Data for travel map screen (everything less place names)
btlsys00 - enemy names (sub-"files" of btlsys.cnk which is a sub-file under SYSTEM.VOL)
btlsys01 - skill names and descriptions
btlsys02 - battle messages
Almost done. Just Z0_5_1 to go and about halfway through.
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28May16: Got the UI stuff working. Need help removing length limit. Working on transcribing quest text from the subs so it can be inserted.
29May16: Quest inserted. Need help removing length limits. Place names, action icons translated.