Mozile uses Document Type Definition (DTD) files and a properties file to allow for internationalization of the fixed toolbar commands and the various XUL windows. There is a DTD file for each XUL file. The MES defining the custom toolbar can also be simply edited to produce a version in the required language. The default language used is American English (en-US).
Error messages and debugging information are always in English and can not be localized, other than by changing the Javascript source files.
The files can be found in the "mozile/locale/en-US/" directory via your local file system. Note that the browser changes the local directory name to "chrome://mozile/locale/mozile.dtd when accessed from Chrome.
DTD files can not be used when text strings are inserted from Javascript. Instead a "properties" file is used. Mozile uses a single properties file.