Tkinter wraps the tcl/tk gui framework.  The file dialogs and messageboxes are provided by tk.  When possible, the file dialogs utilize the native OS file dialogs.  The localization is done the by the OS. If the messageboxes are localized, then I presume the same is true of them also.

tkinter.simpledialog is a tkinter add-on module written in Python, using tk widgets.  SimpleDialog does not have default buttons; they must be supplied in the initialization call.  Dialog does provide default *example* buttons.  The docstring says "override if you do not want the standard buttons".  

Tk does not provide localized [Ok] and [Cancel] buttons.  AFAIK, Python does not provide access to the OS local translations. You will have to provide localization yourself.

A reasonable enhancement request might be to add "ok='OK', cancel='Cancel'" to the signature of Dialog.__init__ so that a user could pass in local translations. These would then be passed on to buttonbox, to be used instead of the current hard-coded strings.