diff --git a/pelican/settings.py b/pelican/settings.py
index f681a182..b963007a 100644
--- a/pelican/settings.py
+++ b/pelican/settings.py
@@ -245,6 +245,22 @@ def get_jinja_environment(settings):
     return settings
 
 
+def _printf_s_to_format_field(printf_string, format_field):
+    """Tries to replace %s with {format_field} in the provided printf_string.
+    Raises ValueError in case of failure.
+    """
+    TEST_STRING = 'PELICAN_PRINTF_S_DEPRECATION'
+    expected = printf_string % TEST_STRING
+
+    result = printf_string.replace('{', '{{').replace('}', '}}') \
+        % '{{{}}}'.format(format_field)
+    if result.format(**{format_field: TEST_STRING}) != expected:
+        raise ValueError('Failed to safely replace %s with {{{}}}'.format(
+            format_field))
+
+    return result
+
+
 def handle_deprecated_settings(settings):
     """Converts deprecated settings and issues warnings. Issues an exception
     if both old and new setting is specified.
@@ -394,10 +410,16 @@ def handle_deprecated_settings(settings):
     for key in ['TRANSLATION_FEED_ATOM',
                 'TRANSLATION_FEED_RSS'
                 ]:
-        if key in settings and '%s' in settings[key]:
+        if settings.get(key) and '%s' in settings[key]:
             logger.warning('%%s usage in %s is deprecated, use {lang} '
-                           'instead. Falling back to default.', key)
-            settings[key] = DEFAULT_CONFIG[key]
+                           'instead.', key)
+            try:
+                settings[key] = _printf_s_to_format_field(
+                    settings[key], 'lang')
+            except ValueError:
+                logger.warning('Failed to convert %%s to {lang} for %s. '
+                               'Falling back to default.', key)
+                settings[key] = DEFAULT_CONFIG[key]
     for key in ['AUTHOR_FEED_ATOM',
                 'AUTHOR_FEED_RSS',
                 'CATEGORY_FEED_ATOM',
@@ -405,10 +427,16 @@ def handle_deprecated_settings(settings):
                 'TAG_FEED_ATOM',
                 'TAG_FEED_RSS',
                 ]:
-        if key in settings and '%s' in settings[key]:
+        if settings.get(key) and '%s' in settings[key]:
             logger.warning('%%s usage in %s is deprecated, use {slug} '
-                           'instead. Falling back to default.', key)
-            settings[key] = DEFAULT_CONFIG[key]
+                           'instead.', key)
+            try:
+                settings[key] = _printf_s_to_format_field(
+                    settings[key], 'slug')
+            except ValueError:
+                logger.warning('Failed to convert %%s to {slug} for %s. '
+                               'Falling back to default.', key)
+                settings[key] = DEFAULT_CONFIG[key]
 
     return settings
 
diff --git a/pelican/tests/test_settings.py b/pelican/tests/test_settings.py
index a92cf09c..21759d40 100644
--- a/pelican/tests/test_settings.py
+++ b/pelican/tests/test_settings.py
@@ -9,6 +9,7 @@ from sys import platform
 
 
 from pelican.settings import (DEFAULT_CONFIG, DEFAULT_THEME,
+                              _printf_s_to_format_field,
                               configure_settings, handle_deprecated_settings,
                               read_settings)
 from pelican.tests.support import unittest
@@ -168,6 +169,14 @@ class TestSettingsConfiguration(unittest.TestCase):
 
         self.assertRaises(Exception, configure_settings, settings)
 
+    def test__printf_s_to_format_field(self):
+        for s in ('%s', '{%s}', '{%s'):
+            option = 'foo/{}/bar.baz'.format(s)
+            result = _printf_s_to_format_field(option, 'slug')
+            expected = option % 'qux'
+            found = result.format(slug='qux')
+            self.assertEqual(expected, found)
+
     def test_deprecated_extra_templates_paths(self):
         settings = self.settings
         settings['EXTRA_TEMPLATES_PATHS'] = ['/foo/bar', '/ha']