Update i18n.js

When looking for a translation, the fallback solution (CurrentLocale -> defaultLocale -> fallbackString) does not work for cms/javascript/CMSMain.Tree.js as localization for this part was changed to short locale names (e.g. de_DE -> de). (Don't know why...)

The original fallback solution will not find a translation for e.g. "Tree.ShowAsList" in the de-language file. For this entry there is also no fallbackString defined, so the menu in the CMS (right click on a page) is broken for each language that does not have the translation included.

I added another fallback level where the short version of the default language (en_US -> us) is searched before falling back to the fallbackstring and then finally giving up...