You will need a text editor that supports editing UTF-8 formatted XML files (you can try free Notepad++).
To create a new translation - browse the languages folder and take the 'English.xml' file as an example. Rename your xml file to the English name of your native language (e.g. Spanish.xml, Chinese.xml).
To update an existing translation - browse the languages folder and find your language, e.g. Spanish.xml
Translate all English strings into your language. Make sure that the file is correctly opened by the Internet Explorer browser.
Always leave %s or %d in the string, it's a replacement pattern required by the program. For example, the string "Program %s uninstalled" will be expanded to "Program Firefox uninstalled" when Firefox is uninstalled. Be careful when escaping HTML characters.
Do not edit LangID of an existent language file. This is done by us using a target="blank" href="http://msdn.microsoft.com/en-us/goglobal/bb964664.aspx">Microsoft languages table.
Uninstall Tool User Guide
This topic is a part of Uninstall Tool Support Articles.