Proper i18n solution for "Files for direct translation"

[RanYanivHartstein]

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.

[nbachiyski]

The patch above wraps strings, needing direct translation, in special comments.

Using the ​not-gettexted.php tool these strings can be extracted in POT files or replaced with translations from a MO file.

[ryan]

(In [8168]) Mark string that cannot be gettexted. Props nbachiyski. fixes #7074