The Codex page Files for Direct Translation lists all the core files that translators have to edit directly for a complete localization.
These are mostly error messages in files that load before gettext is initialized. Some of them are loaded before wp_config.php and therefore can't tell yet what the proper locale is. One gets loaded when wp_config.php is *missing*, so there is no way of knowing what the proper locale is (this is discussed in ticket #6132).
A system needs to be put in place to allow localizing these strings without editing core files, or at least of editing just one core file in a specific location. This should be independent of the gettext system. For e.g., a single file place in the root folder that sets these strings as variables to be used in the various other files that need them.
