1. GNU C Library: National Language (locale) data [support] Machine-readable data files, shared objects and programs used by the C library for localization (l10n) and internationalization (i18n) support. This package contains the libc.mo i18n files, plus tools to generate locale definitions from source files (included in this package). It allows you to customize which definitions actually get generated. This is a savings over how this package used to be, where all locales were generated by default. This created a package that unpacked to an excess of 30 megs. From Debian 3.0r0 APT

2. These are the base files for language localization. You also need to install the specific locales-?? for the language(s) you want. Then the user need to set the LANG variable to their preferred language in their ~/.profile configuration file. From Mandrake 9.0 RPM