As you probably already know, WordPress supports language packs. Translation files that reside in
wp-content/languages. Those are used to translate all the strings the user sees in WordPress.
WordPress also allows translation teams on WordPress.org to provide localized release packages. These already come with the translation files included, plus a bit more. They can translate the readme file or bundle locale-specific functionality (e.g. custom plugins) to improve the UX for that locale.
For example, the Japanese package contains the WP Multibyte Batch plugin and the readme is translated into Japanese as well. This is something the language packs cannot handle, as the readme is a simple HTML file. Similarly, the German package contains the original readme and a translated one.