diff --git a/src/testdir/test_environ.vim b/src/testdir/test_environ.vim
index d9f99f0..dd34983 100644
--- a/src/testdir/test_environ.vim
+++ b/src/testdir/test_environ.vim
@@ -2,6 +2,8 @@
 
 scriptencoding utf-8
 
+source check.vim
+
 func Test_environ()
   unlet! $TESTENV
   call assert_equal(0, has_key(environ(), 'TESTENV'))
@@ -45,4 +47,23 @@
   call assert_equal('', result)
 endfunc
 
+func Test_mac_locale()
+  CheckFeature osxdarwin
+
+  " If $LANG is not set then the system locale will be used.
+  " Run Vim after unsetting all the locale environmental vars, and capture the
+  " output of :lang.
+  let lang_results = system("unset LANG; unset LC_MESSAGES; " ..
+            \ shellescape(v:progpath) ..
+            \ " --clean -esX -c 'redir @a' -c 'lang' -c 'put a' -c 'print' -c 'qa!' ")
+
+  " Check that:
+  " 1. The locale is the form of <locale>.UTF-8.
+  " 2. Check that fourth item (LC_NUMERIC) is properly set to "C".
+  " Example match: "en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8"
+  call assert_match('"\([a-zA-Z_]\+\.UTF-8/\)\{3}C\(/[a-zA-Z_]\+\.UTF-8\)\{2}"',
+        \ lang_results,
+        \ "Default locale should have UTF-8 encoding set, and LC_NUMERIC set to 'C'")
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab