well, gnome creates folders in whatever language you installed with, tho it can translate the file manager's sidebar items, I think...
so a change of locale means you'd still have the old dir left, and show as old name everywhere except the special shortcuts
Only in the search index... If you try to install your application in "C:\Program Files\" on a non-english version of windows, you'll have a bad time...
I use Swedish Windows, and here "C:\Program\" (the localization) and "C:\Program Files\" both work fine and go to the same place.
Not since Vista. These days, you'll always have "Program Files", and if you have the OS configured for a different locale, it'll create a hard link for the localized name. The Windows shell (including the standard file picker dialogs etc) will also use the localized name preferentially whenever it has to display a path that contains one of those folders, but still allows verbatim paths to be used wherever text input is allowed.