diff --git a/src/test/regress/pg_regress.c b/src/test/regress/pg_regress.c
index e8c644b..e55835e 100644
--- a/src/test/regress/pg_regress.c
+++ b/src/test/regress/pg_regress.c
@@ -790,9 +790,16 @@ initialize_environment(void)
 		unsetenv("LC_NUMERIC");
 		unsetenv("LC_TIME");
 		unsetenv("LANG");
-		/* On Windows the default locale cannot be English, so force it */
-#if defined(WIN32) || defined(__CYGWIN__)
-		putenv("LANG=en");
+		/*
+		 * Most platforms have adopted the POSIX locale as their
+		 * implementation-defined default locale.  Exceptions include native
+		 * Windows, Darwin with --enable-nls, and Cygwin with --enable-nls.
+		 * (Use of --enable-nls matters because libintl replaces setlocale().)
+		 * Also, PostgreSQL does not support Darwin with locale environment
+		 * variables unset; see PostmasterMain().
+		 */
+#if defined(WIN32) || defined(__CYGWIN__) || defined(__darwin__)
+		putenv("LANG=C");
 #endif
 	}