Yes, permanent. Since 1.3, there's Locale/Locale.js instead of Core/Lang.js.
If you're referring to things on the master branch, things can always change, it'd be saver using a tag, like: https://raw.github.com/mootools/mootools-more/1.2.5.1/Source/Core/Lang.js The current master-version is now at: https://raw.github.com/mootools/mootools-more/master/Source/Locale/Locale.js The 1.3.2.1-version: https://raw.github.com/mootools/mootools-more/1.3.2.1/Source/Locale/Locale.js
