[XeTeX] Command \providelength already defined

[Arthur Reutenauer]

> It may be that polyglossia is more careful about re-defining commands.

  As I said, polyglossia isn't the package that defines that command,
neither is dialogue.  It could be interesting to investigate in more
details but I really don't have the time right now.  (And last time I
did that kind of exercise it turned out that the package author had been
dead for ten years and nobody had volunteered to take it over, so you'll
have to forgive me for focusing on other things.)

	Arthur