diff --git a/libcxx/test/std/containers/container.adaptors/container.adaptors.format/format.functions.tests.h b/libcxx/test/std/containers/container.adaptors/container.adaptors.format/format.functions.tests.h
--- a/libcxx/test/std/containers/container.adaptors/container.adaptors.format/format.functions.tests.h
+++ b/libcxx/test/std/containers/container.adaptors/container.adaptors.format/format.functions.tests.h
@@ -600,16 +600,19 @@
   // *** locale-specific form ***
   check(SV("[-42.5, 0, 1.25, 42.5]"), SV("{::L}"), input); // does not require locales present
 #ifndef TEST_HAS_NO_LOCALIZATION
-// TODO FMT Enable with locale testing active
-#  if 0
+#  ifdef TEST_HAS_LOCALE_fr_FR_UTF_8
   std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
   check(SV("[-42,5, 0, 1,25, 42,5]"), SV("{::L}"), input);
+#  else
+#    error XXX
+#  endif
 
+#  ifdef TEST_HAS_LOCALE_en_US_UTF_8
   std::locale::global(std::locale(LOCALE_en_US_UTF_8));
   check(SV("[-42.5, 0, 1.25, 42.5]"), SV("{::L}"), input);
+#  endif
 
   std::locale::global(std::locale::classic());
-#  endif
 #endif // TEST_HAS_NO_LOCALIZATION
 
   // *** type ***
diff --git a/libcxx/test/std/utilities/format/format.range/format.range.fmtset/format.functions.tests.h b/libcxx/test/std/utilities/format/format.range/format.range.fmtset/format.functions.tests.h
--- a/libcxx/test/std/utilities/format/format.range/format.range.fmtset/format.functions.tests.h
+++ b/libcxx/test/std/utilities/format/format.range/format.range.fmtset/format.functions.tests.h
@@ -776,16 +776,17 @@
   // *** locale-specific form ***
   check(SV("{-42.5, 0, 1.25, 42.5}"), SV("{::L}"), input); // does not require locales present
 #ifndef TEST_HAS_NO_LOCALIZATION
-// TODO FMT Enable with locale testing active
-#  if 0
+#  ifdef TEST_HAS_LOCALE_fr_FR_UTF_8
   std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
   check(SV("{-42,5, 0, 1,25, 42,5}"), SV("{::L}"), input);
+#  endif
 
+#  ifdef TEST_HAS_LOCALE_en_US_UTF_8
   std::locale::global(std::locale(LOCALE_en_US_UTF_8));
   check(SV("{-42.5, 0, 1.25, 42.5}"), SV("{::L}"), input);
+#  endif
 
   std::locale::global(std::locale::classic());
-#  endif
 #endif // TEST_HAS_NO_LOCALIZATION
 
   // *** type ***
diff --git a/libcxx/test/std/utilities/format/format.range/format.range.formatter/format.functions.tests.h b/libcxx/test/std/utilities/format/format.range/format.range.formatter/format.functions.tests.h
--- a/libcxx/test/std/utilities/format/format.range/format.range.formatter/format.functions.tests.h
+++ b/libcxx/test/std/utilities/format/format.range/format.range.formatter/format.functions.tests.h
@@ -587,16 +587,17 @@
   // *** locale-specific form ***
   check(SV("[-42.5, 0, 1.25, 42.5]"), SV("{::L}"), input); // does not require locales present
 #ifndef TEST_HAS_NO_LOCALIZATION
-// TODO FMT Enable with locale testing active
-#  if 0
+#  ifdef TEST_HAS_LOCALE_fr_FR_UTF_8
   std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
   check(SV("[-42,5, 0, 1,25, 42,5]"), SV("{::L}"), input);
+#  endif
 
+#  ifdef TEST_HAS_LOCALE_en_US_UTF_8
   std::locale::global(std::locale(LOCALE_en_US_UTF_8));
   check(SV("[-42.5, 0, 1.25, 42.5]"), SV("{::L}"), input);
+#  endif
 
   std::locale::global(std::locale::classic());
-#  endif
 #endif // TEST_HAS_NO_LOCALIZATION
 
   // *** type ***
diff --git a/libcxx/test/support/test_macros.h b/libcxx/test/support/test_macros.h
--- a/libcxx/test/support/test_macros.h
+++ b/libcxx/test/support/test_macros.h
@@ -7,8 +7,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef SUPPORT_TEST_MACROS_HPP
-#define SUPPORT_TEST_MACROS_HPP
+#ifndef SUPPORT_TEST_MACROS_H
+#define SUPPORT_TEST_MACROS_H
 
 // Attempt to get STL specific macros like _LIBCPP_VERSION using the most
 // minimal header possible. If we're testing libc++, we should use `<__config>`.
@@ -23,6 +23,12 @@
 #include <ciso646>
 #endif
 
+#ifdef __has_include
+#  if __has_include("config_site.h")
+#    include "config_site.h"
+#  endif
+#endif
+
 #define TEST_STRINGIZE_IMPL(x) #x
 #define TEST_STRINGIZE(x) TEST_STRINGIZE_IMPL(x)
 
@@ -437,4 +443,4 @@
 #  define TEST_SHORT_WCHAR
 #endif
 
-#endif // SUPPORT_TEST_MACROS_HPP
+#endif // SUPPORT_TEST_MACROS_H
diff --git a/libcxx/utils/libcxx/test/dsl.py b/libcxx/utils/libcxx/test/dsl.py
--- a/libcxx/utils/libcxx/test/dsl.py
+++ b/libcxx/utils/libcxx/test/dsl.py
@@ -505,6 +505,35 @@
     return 'add substitution {} = {}'.format(self._key, self._getSub(config))
 
 
+class AddDefine(ConfigAction):
+  """
+  This action adds the given define to the test suite's config_site.
+
+  Currently this only defines macros without a way to specify the macro's
+  value, since there is no usecase for that feature.
+
+  """
+  def __init__(self, name):
+    self._name = name
+
+  def initializeConfig(self, config):
+    config.defines = []
+    config.config_site_root = os.path.join(config.test_exec_root, 'support')
+    if not os.path.exists(config.config_site_root):
+      os.makedirs(config.config_site_root)
+    AddCompileFlag(f"-I {config.config_site_root}").applyTo(config)
+
+  def applyTo(self, config):
+    name = self._name
+    if not hasattr(config, 'defines'):
+      self.initializeConfig(config)
+    config.defines.append(name)
+
+  def pretty(self, config, litParams):
+    return f"add define {self._name}"
+
+
+
 class Feature(object):
   """
   Represents a Lit available feature that is enabled whenever it is supported.
diff --git a/libcxx/utils/libcxx/test/features.py b/libcxx/utils/libcxx/test/features.py
--- a/libcxx/utils/libcxx/test/features.py
+++ b/libcxx/utils/libcxx/test/features.py
@@ -240,7 +240,8 @@
   # Note: Using alts directly in the lambda body here will bind it to the value at the
   # end of the loop. Assigning it to a default argument works around this issue.
   DEFAULT_FEATURES.append(Feature(name='locale.{}'.format(locale),
-                                  when=lambda cfg, alts=alts: hasAnyLocale(cfg, alts)))
+                                  when=lambda cfg, alts=alts: hasAnyLocale(cfg, alts),
+                                  actions=[AddDefine('TEST_HAS_LOCALE_{}'.format(re.sub("\W", "_", locale)))]))
 
 
 # Add features representing the target platform name: darwin, linux, windows, etc...
diff --git a/libcxx/utils/libcxx/test/newconfig.py b/libcxx/utils/libcxx/test/newconfig.py
--- a/libcxx/utils/libcxx/test/newconfig.py
+++ b/libcxx/utils/libcxx/test/newconfig.py
@@ -15,6 +15,31 @@
       return replacement
   raise ValueError('Substitution {} is not in the config.'.format(substitution))
 
+CONFIG_SITE_TEMPLATE = """
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SUPPORT_TEST_CONFIG_SITE_H
+#define SUPPORT_TEST_CONFIG_SITE_H
+
+{defines}
+
+#endif // SUPPORT_TEST_CONFIG_SITE_H
+"""
+
+def _writeConfigSite(config):
+  file = open(os.path.join(config.config_site_root, 'config_site.h'), "w")
+  file.write(CONFIG_SITE_TEMPLATE.format(defines = '\n'.join(
+      [
+          "#define {}".format(d) for d in config.defines
+      ])))
+  file.close()
+
 def configure(parameters, features, config, lit_config):
   note = lambda s: lit_config.note("({}) {}".format(config.name, s))
   config.environment = dict(os.environ)
@@ -41,9 +66,16 @@
           action.pretty(config, lit_config.params),
           feature.pretty(config)))
 
+  if hasattr(config, 'defines'):
+    _writeConfigSite(config)
+
   # Print the basic substitutions
   for sub in ('%{cxx}', '%{flags}', '%{compile_flags}', '%{link_flags}', '%{exec}'):
     note("Using {} substitution: '{}'".format(sub, _getSubstitution(sub, config)))
 
   # Print all available features
   note("All available features: {}".format(', '.join(config.available_features)))
+
+  # Print all available defines
+  if hasattr(config, 'defines'):
+    note("All available defines: {}".format(', '.join(config.defines)))