diff --git a/application/Utils.php b/application/Utils.php
index 48d474155..4d284ccbc 100644
--- a/application/Utils.php
+++ b/application/Utils.php
@@ -322,7 +322,7 @@ function format_date($date, $time = true, $intl = true)
         return $date->format($format);
     }
     $formatter = new IntlDateFormatter(
-        setlocale(LC_TIME, 0),
+        get_locale(LC_TIME),
         IntlDateFormatter::LONG,
         $time ? IntlDateFormatter::LONG : IntlDateFormatter::NONE
     );
@@ -503,3 +503,23 @@ function exception2text(Throwable $e): string
 {
     return $e->getMessage() . PHP_EOL . $e->getFile() . $e->getLine() . PHP_EOL . $e->getTraceAsString();
 }
+
+/**
+ * Get the current locale, overrides 'C' locale which is no longer compatible with PHP-intl
+ *
+ * @param int $category Category of the locale (LC_CTYPE, LC_NUMERIC, LC_TIME, LC_COLLATE, LC_MONETARY, LC_ALL)
+ *
+ * @return string|false The locale, or false if not found.
+ *
+ * @see https://github.com/php/php-src/issues/12561
+ */
+function get_locale(int $category = LC_CTYPE)
+{
+    $locale = setlocale($category, 0);
+
+    if ($locale === 'C' || startsWith($locale, 'C.')) {
+        $locale = 'en_US.utf8'; // failback
+    }
+
+    return $locale;
+}
diff --git a/application/front/controller/visitor/FeedController.php b/application/front/controller/visitor/FeedController.php
index 622fe767e..88a455662 100644
--- a/application/front/controller/visitor/FeedController.php
+++ b/application/front/controller/visitor/FeedController.php
@@ -38,7 +38,7 @@ protected function processRequest(string $feedType, Request $request, Response $
         }
 
         // Generate data.
-        $this->container->get('feedBuilder')->setLocale(strtolower(setlocale(LC_COLLATE, 0)));
+        $this->container->get('feedBuilder')->setLocale(strtolower(get_locale(LC_COLLATE)));
         $this->container->get('feedBuilder')->setHideDates($this->container->get('conf')
             ->get('privacy.hide_timestamps', false));
         $this->container->get('feedBuilder')->setUsePermalinks(
diff --git a/application/http/HttpUtils.php b/application/http/HttpUtils.php
index 4bde1d5b8..a63b1fb45 100644
--- a/application/http/HttpUtils.php
+++ b/application/http/HttpUtils.php
@@ -55,7 +55,7 @@ function get_http_response(
         'Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:45.0)'
         . ' Gecko/20100101 Firefox/45.0';
     $acceptLanguage =
-        substr(setlocale(LC_COLLATE, 0), 0, 2) . ',en-US;q=0.7,en;q=0.3';
+        substr(get_locale(LC_COLLATE), 0, 2) . ',en-US;q=0.7,en;q=0.3';
     $maxRedirs = 3;
 
     if (!function_exists('curl_init')) {
diff --git a/index.php b/index.php
index def5b0889..560eb14d0 100644
--- a/index.php
+++ b/index.php
@@ -83,7 +83,7 @@
     autoLocale($_SERVER['HTTP_ACCEPT_LANGUAGE']);
 }
 
-new Languages(setlocale(LC_MESSAGES, 0), $conf);
+new Languages(get_locale(LC_MESSAGES), $conf);
 
 $conf->setEmpty('general.timezone', date_default_timezone_get());
 $conf->setEmpty('general.title', t('Shared bookmarks on ') . escape(index_url($_SERVER)));
diff --git a/tests/languages/de/UtilsDeTest.php b/tests/languages/de/UtilsDeTest.php
index d7a16eaca..014320d18 100644
--- a/tests/languages/de/UtilsDeTest.php
+++ b/tests/languages/de/UtilsDeTest.php
@@ -20,7 +20,7 @@ public function testIntlDateFormatter()
      */
     public function testDateFormat()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         autoLocale('de-de');
         $date = DateTime::createFromFormat('Ymd_His', '20170102_201112');
         $this->assertRegExp('/2\. Januar 2017 (um )?20:11:12 GMT\+0?3(:00)?/', format_date($date, true, true));
@@ -32,7 +32,7 @@ public function testDateFormat()
      */
     public function testDateFormatNoTime()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         autoLocale('de-de');
         $date = DateTime::createFromFormat('Ymd_His', '20170102_201112');
         $this->assertRegExp('/2\. Januar 2017/', format_date($date, false, true));
@@ -62,10 +62,10 @@ public function testDateFormatDefaultNoTime()
      */
     public function testAutoLocaleValid()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         $header = 'en-us';
         autoLocale($header);
-        $this->assertEquals('en_US.utf8', setlocale(LC_ALL, 0));
+        $this->assertEquals('en_US.utf8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -75,10 +75,10 @@ public function testAutoLocaleValid()
      */
     public function testAutoLocaleValidAlternative()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         $header = 'en_US.UTF-8';
         autoLocale($header);
-        $this->assertEquals('en_US.utf8', setlocale(LC_ALL, 0));
+        $this->assertEquals('en_US.utf8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -88,10 +88,10 @@ public function testAutoLocaleValidAlternative()
      */
     public function testAutoLocaleMultipleFirstValid()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         $header = 'en-us,de-de';
         autoLocale($header);
-        $this->assertEquals('en_US.utf8', setlocale(LC_ALL, 0));
+        $this->assertEquals('en_US.utf8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -101,10 +101,10 @@ public function testAutoLocaleMultipleFirstValid()
      */
     public function testAutoLocaleMultipleSecondAvailable()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         $header = 'mgg_IN,fr-fr';
         autoLocale($header);
-        $this->assertEquals('fr_FR.utf8', setlocale(LC_ALL, 0));
+        $this->assertEquals('fr_FR.utf8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -114,9 +114,9 @@ public function testAutoLocaleMultipleSecondAvailable()
      */
     public function testAutoLocaleBlank()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         autoLocale('');
-        $this->assertEquals('en_US.UTF-8', setlocale(LC_ALL, 0));
+        $this->assertEquals('en_US.UTF-8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -126,9 +126,9 @@ public function testAutoLocaleBlank()
      */
     public function testAutoLocaleUnavailable()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         autoLocale('mgg_IN');
-        $this->assertEquals('en_US.UTF-8', setlocale(LC_ALL, 0));
+        $this->assertEquals('en_US.UTF-8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
diff --git a/tests/languages/en/UtilsEnTest.php b/tests/languages/en/UtilsEnTest.php
index a59f989b5..af2226a2d 100644
--- a/tests/languages/en/UtilsEnTest.php
+++ b/tests/languages/en/UtilsEnTest.php
@@ -20,7 +20,7 @@ public function testIntlDateFormatter()
      */
     public function testDateFormat()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         autoLocale('en_US.UTF-8');
         $date = DateTime::createFromFormat('Ymd_His', '20170102_201112');
         // Replace No-Break-Space with simple space
@@ -34,7 +34,7 @@ public function testDateFormat()
      */
     public function testDateFormatNoTime()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         autoLocale('en_US.UTF-8');
         $date = DateTime::createFromFormat('Ymd_His', '20170102_201112');
         $this->assertRegExp('/January 2, 2017/', format_date($date, false, true));
@@ -64,10 +64,10 @@ public function testDateFormatDefaultNoTime()
      */
     public function testAutoLocaleValid()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         $header = 'de-de';
         autoLocale($header);
-        $this->assertEquals('de_DE.utf8', setlocale(LC_ALL, 0));
+        $this->assertEquals('de_DE.utf8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -77,10 +77,10 @@ public function testAutoLocaleValid()
      */
     public function testAutoLocaleValidAlternative()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         $header = 'de_de.UTF8';
         autoLocale($header);
-        $this->assertEquals('de_DE.utf8', setlocale(LC_ALL, 0));
+        $this->assertEquals('de_DE.utf8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -90,10 +90,10 @@ public function testAutoLocaleValidAlternative()
      */
     public function testAutoLocaleMultipleFirstValid()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         $header = 'de-de;en-us';
         autoLocale($header);
-        $this->assertEquals('de_DE.utf8', setlocale(LC_ALL, 0));
+        $this->assertEquals('de_DE.utf8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -103,10 +103,10 @@ public function testAutoLocaleMultipleFirstValid()
      */
     public function testAutoLocaleMultipleSecondAvailable()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         $header = 'mgg_IN,fr-fr';
         autoLocale($header);
-        $this->assertEquals('fr_FR.utf8', setlocale(LC_ALL, 0));
+        $this->assertEquals('fr_FR.utf8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -116,9 +116,9 @@ public function testAutoLocaleMultipleSecondAvailable()
      */
     public function testAutoLocaleBlank()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         autoLocale('');
-        $this->assertEquals('en_US.UTF-8', setlocale(LC_ALL, 0));
+        $this->assertEquals('en_US.UTF-8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -128,9 +128,9 @@ public function testAutoLocaleBlank()
      */
     public function testAutoLocaleUnavailable()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         autoLocale('mgg_IN');
-        $this->assertEquals('en_US.UTF-8', setlocale(LC_ALL, 0));
+        $this->assertEquals('en_US.UTF-8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
diff --git a/tests/languages/fr/UtilsFrTest.php b/tests/languages/fr/UtilsFrTest.php
index 799fa5c96..a6ae0e4b1 100644
--- a/tests/languages/fr/UtilsFrTest.php
+++ b/tests/languages/fr/UtilsFrTest.php
@@ -20,7 +20,7 @@ public function testIntlDateFormatter()
      */
     public function testDateFormat()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         autoLocale('fr-fr');
         $date = DateTime::createFromFormat('Ymd_His', '20170102_201112');
         $this->assertRegExp('/2 janvier 2017 (à )?20:11:12 UTC\+0?3(:00)?/', format_date($date));
@@ -32,7 +32,7 @@ public function testDateFormat()
      */
     public function testDateFormatNoTime()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         autoLocale('fr-fr');
         $date = DateTime::createFromFormat('Ymd_His', '20170102_201112');
         $this->assertRegExp('/2 janvier 2017/', format_date($date, false, true));
@@ -62,10 +62,10 @@ public function testDateFormatDefaultNoTime()
      */
     public function testAutoLocaleValid()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         $header = 'de-de';
         autoLocale($header);
-        $this->assertEquals('de_DE.utf8', setlocale(LC_ALL, 0));
+        $this->assertEquals('de_DE.utf8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -75,10 +75,10 @@ public function testAutoLocaleValid()
      */
     public function testAutoLocaleValidAlternative()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         $header = 'de_de.UTF8';
         autoLocale($header);
-        $this->assertEquals('de_DE.utf8', setlocale(LC_ALL, 0));
+        $this->assertEquals('de_DE.utf8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -88,10 +88,10 @@ public function testAutoLocaleValidAlternative()
      */
     public function testAutoLocaleMultipleFirstValid()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         $header = 'de-de;en-us';
         autoLocale($header);
-        $this->assertEquals('de_DE.utf8', setlocale(LC_ALL, 0));
+        $this->assertEquals('de_DE.utf8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -101,10 +101,10 @@ public function testAutoLocaleMultipleFirstValid()
      */
     public function testAutoLocaleMultipleSecondAvailable()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         $header = 'mgg_IN,de-de';
         autoLocale($header);
-        $this->assertEquals('de_DE.utf8', setlocale(LC_ALL, 0));
+        $this->assertEquals('de_DE.utf8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -114,9 +114,9 @@ public function testAutoLocaleMultipleSecondAvailable()
      */
     public function testAutoLocaleBlank()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         autoLocale('');
-        $this->assertEquals('en_US.UTF-8', setlocale(LC_ALL, 0));
+        $this->assertEquals('en_US.UTF-8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }
@@ -126,9 +126,9 @@ public function testAutoLocaleBlank()
      */
     public function testAutoLocaleUnavailable()
     {
-        $current = setlocale(LC_ALL, 0);
+        $current = get_locale(LC_ALL);
         autoLocale('mgg_IN');
-        $this->assertEquals('en_US.UTF-8', setlocale(LC_ALL, 0));
+        $this->assertEquals('en_US.UTF-8', get_locale(LC_ALL));
 
         setlocale(LC_ALL, $current);
     }