The Arabic word for dog is al-kalb (الكلب), but when I try to insert this
word into an otherwise left-to-right document, Emacs renders the characters
of al-kalb backwards, forming nonsense.

System:

$ specs emacs brew os
specs --version
0.21

emacs --version
GNU Emacs 24.5.1
Copyright (C) 2015 Free Software Foundation, Inc.
GNU Emacs comes with ABSOLUTELY NO WARRANTY.
You may redistribute copies of Emacs
under the terms of the GNU General Public License.
For more information about these matters, see the file named COPYING.

brew --version
Homebrew 0.9.5 (git revision 1fc4; last commit 2016-02-09)

system_profiler SPSoftwareDataType | grep 'System Version'
    System Software Overview:
      System Version: OS X 10.11.3 (15D21)
      System Integrity Protection: Enabled

-- 
Cheers,
Andrew