Shared components used by Firefox and other Mozilla software, including handling of Web content; Gecko, HTML, CSS, layout, DOM, scripts, images, networking, etc. Issues with web page layout probably go here, while Firefox user interface issues belong in the Firefox product. (More info)
Internationalization is the process of designing and developing a software product to function in multiple locales. This process involves identifying the locales that must be supported, designing features which support those locales, and writing the code needed.
unixcharset.properties is for old UNIX (or libc doesn't support nl_langinfo(CODESET).). And current Gecko doesn't support for OpenVMS, SCO and HP-UX 10. So we don't need locale.OSTYPE support in unixcharset.properties.