l2.ini – contains information about the server and client settings (including the IP address of the server the game is trying to connect to). The file may implement features such as borderless windowed mode, increased nickname display distance, etc. It is a component of windowed-mode patches.
user.ini – hotkeys, zoom (some of your personal settings are also stored there).
Option.ini – client configuration file. A component of windowed-mode patches.
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eula-e.dat – license agreement.
Three files responsible for drop/spoil and Russian skill descriptions, as well as chests: npcgrp.dat, Skillgrp.dat, SkillName-e.dat.
npcname-e.dat – controls the display of mob names. Shown above the head: aggro status and mob level. (Also responsible for displaying drops, spoil, and mob skills.) A component of the drop/spoil patch.
Skillgrp.dat – responsible for linking a skill to its visual representation and for the NPC/Mob information panel. A component of the drop/spoil patch.
SkillName-e.dat – skill names and their descriptions. A component of the drop/spoil patch.
sysstring-e.dat – stores information from the login menu, character menu, etc.; button names (basically almost the entire interface).
systemmsg-e.dat – responsible for chat (I recommend downloading my chat patch, it’s great ^^).
env.int – controls the color of weapon enchantments.
itemname-e.dat – responsible for the names of all items in the game, the number of crystals an item breaks into, and the price at which it can be sold to a shop.
questname-e.dat – responsible for quest translations.
chargrp.dat – responsible for player nudity. You can overdo it here—there are “18+” patches on the internet, almost pornographic.
actionname-e.dat – responsible for localization of the Alt-C actions menu.
castlename-e.dat – responsible for the names of castles and clan halls.
classinfo-e.dat – responsible for information about different classes, shown during character creation.
gametip-e.dat – responsible for helpful tips during game loading.
servername-e.dat – responsible for server names.