Mozilla / Firefox
Mozilla and Firefox has very odd file-system (.dtd and .properties format of XML) that’s may not be suitable for l10n people. There is a tool called ‘Mozilla Translator’ on sourforge but it’s in Java and seems not supporting Unicode. I need to fix this else I have to use moz2po and po2moz tools (Thanks to http://translate.sourceforge.net). l10n people should consider that browser is the most popular s/w right now and must give priority to it. Mozilla has good-easy to understable structure of installable package (xpi) that let you create them, extensions and your language pack. It has amazing collection of themes and extensions.