GooTool's code is fully internationalised and it is easy to add new languages. However I am not personally fluent in anything else. If you are interested in working on a translation, leave a comment on this page, and I will send you the file for translation once GooTool is in a stable state.

Yes, hopefully. GooTool is written in pure Java and should be easy to port.

There's currently a Linux version on the download page if you're running under Wine or PlayOnLinux.

The Mac version will be underway as soon as I get back access to a Mac. Some issues may exist with things like the directory path separator, automatic location of WorldOfGoo.exe and prof2.dat, and a new installer/JRE downloader will be needed. I also understand that the encryption of the Mac files is rather different (XOR instead of AES).

The proper Linux version will be out... whenever World of Goo is out for Linux.

Currently it is impossible to add a chapter to the game without causing a crash. When World of Goo supports it, so will GooTool.

An addin is a generic extension to World of Goo that can be installed by GooTool. There are currently two types of addin:

Mod

A generic addin that extends World of Goo, generally by modifying existing functionality or providing new functionality such as new ball types.

Level

An addin that specifically creates a new level within World of Goo. Since modding isn't officially supported yet, new levels are added to Chapter 5.

It is a tool for Goo.