Editing the various language strings and object names is the easiest thing to customize. Please note that this kind of data is kept outside the "data" directory in an directory of it's own. It is named "locales" and can be found in the root of your project d13 directory.
Locales Directories
Each language available in your game is located inside a directory of it's own. You do not have to edit any other configuration or settings in another file. The presence of the directory is enough. This directory should be named after the abbreviated language tag (en, ru, de, it, fr etc.). The engine will automatically take all directories into considerance. Each directory contains three files: userinterface, blockwords and gamelang. All three files must be present in each language directory and must be free of any errors, otherwise it will break the game.
Language Files
The language files each contain a huge array with string entries (like: $ui['won']='won';). You may neither add new entries nor remove existing entries or it will break the game. Whenever you edit an entry, make sure to only edit the righthand entry (thats the actual language string) and not the lefthand entry (thats the variable name).
Example: Say you want to change $ui['won']='won'; into german, that would mean to call it now $ui['won']='gewonnen'; As you can see, the lefthand part stays the same.
d13_userinterface.locale
This contains all user interface related words and sentences. Do not delete any of them, adding new ones will have no effect unless you add them somewhere to the code or templates as well.
d13_blockwords.locale
This file contains a list of bad words, and it should be the only place in your project where you will encounter vulgar language. Each word you state here will be blocked from usernames and messages sent via the game. This means if you add the word "idiot" to the list, no user will be able to register an account called "idiot". Furthermore no user will be able to send a message to another user that contains the word "idiot".
d13_gamelang.locale
This is the heart of the language system, it contains names and descriptions for all objects in the game. It is important to add a new line whenever you add a new object to your game as well. For example the default game data features resources 0 to 5 (six total), if you add resource ID 6, you must add another entry in the resources array section as well, otherwise it will break your game.
Note: As a general note, you should be careful not to break the arrays by keeping all the commas, brackets and other syntax parts intact.