Q:

Passing Alt+letter keybindings through to OSX?

Is there a way to tell Emacs to ignore a certain key binding and pass it to the OS unmodified? Eg. I am typing German from time to time and use Alt+u o to type ö in OS X apps.
With Alt set to Meta I would like to tell emacs to ignore the M-u binding, so that I can use M-u o to enter ö as in other applications. (global-unset-key (kbd "M-u")) does remove the binding from emacs, but pressing M-u does nothing, instead of adding an umlaut to the next pressed character.

A:

When Emacs is built --with-ns for OSX operating systems, the user has configuration options to disable the left and/or right meta key to achieve default OS behavior:  ns-alternate-modifier for the left alt/option key, and ns-right-alternate-modifier for right alt/option key. 
I keep the left alt/option key as meta and disable the right alt/option key within Emacs so that I can write in foreign languages (taking advantage of default OS behavior with the right alt/option key).  Here are my settings for Emacs for OSX and Windows.
(cond
  ((eq system-type 'darwin)
    (setq ns-alternate-modifier 'meta)
    (setq ns-right-alternate-modifier 'none)
    (setq ns-command-modifier 'super)
    (setq ns-right-command-modifier 'left)
    (setq ns-control-modifier 'control)
    (setq ns-right-control-modifier 'left)
    (setq ns-function-modifier 'none))
  ((eq system-type 'windows-nt)
    (setq w32-pass-lwindow-to-system nil)
    (setq w32-pass-rwindow-to-system nil)
    (setq w32-lwindow-modifier 'super)
    (setq w32-rwindow-modifier 'super)))

