pkp#7512 Give preference to the server preferred locale if there's no…

… match with the Accept-Language header

classes/i18n/Locale.php

@@ -132,11 +132,10 @@ public function getLocale(): string
             return $this->locale;
         }
         $request = $this->_getRequest();
-        $supportedLocales = array_values($this->_getSupportedLocales());
         $locale = $request->getUserVar('setLocale')
             ?: $request->getSession()->get('currentLocale')
             ?: $request->getCookieVar('currentLocale')
-            ?: $request->getPreferredLanguage(count($supportedLocales) ? $supportedLocales : [LocaleInterface::DEFAULT_LOCALE]);
+            ?: $this->_getPreferredLocale();
         $this->setLocale($locale);
         return $this->locale;
     }
@@ -584,4 +583,19 @@ private function _getSubmissionLocaleNames(): array
                 ->toArray());
         })();
     }
+
+    /**
+     * Retrieve the preferred user locale from our supported locales using the Accept-Language header
+     * If there's no match, it falls back to the server's primary locale
+     */
+    private function _getPreferredLocale(): ?string
+    {
+        $serverPreference = $this->getPrimaryLocale() ?: LocaleInterface::DEFAULT_LOCALE;
+        $supportedLocales = array_values($this->_getSupportedLocales());
+        // Move the server preference to the top, in case the user preference doesn't match with the supported locales, the server one will be picked
+        if (is_int($index = array_search($serverPreference, $supportedLocales))) {
+            array_splice($supportedLocales, $index, 1);
+        }
+        return $this->_getRequest()->getPreferredLanguage([$serverPreference, ...$supportedLocales]);
+    }
 }