diff --git a/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/MimeTypes.java b/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/MimeTypes.java
index 8cc493092f53..ad7d4991c0ef 100644
--- a/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/MimeTypes.java
+++ b/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/MimeTypes.java
@@ -18,12 +18,14 @@
 import java.io.InputStreamReader;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Locale;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Properties;
+import java.util.Set;
 
 import org.eclipse.jetty.util.FileID;
 import org.eclipse.jetty.util.Index;
@@ -37,6 +39,12 @@
 public class MimeTypes
 {
     static final  Logger LOG = LoggerFactory.getLogger(MimeTypes.class);
+    private static final Set<Locale> KNOWN_LOCALES = Set.copyOf(Arrays.asList(Locale.getAvailableLocales()));
+
+    public static boolean isKnownLocale(Locale locale)
+    {
+        return KNOWN_LOCALES.contains(locale);
+    }
 
     /** Enumeration of predefined MimeTypes. This is not exhaustive */
     public enum Type
diff --git a/jetty-core/jetty-server/src/main/java/org/eclipse/jetty/server/Request.java b/jetty-core/jetty-server/src/main/java/org/eclipse/jetty/server/Request.java
index ae6a849c1b4a..2dbf39788887 100644
--- a/jetty-core/jetty-server/src/main/java/org/eclipse/jetty/server/Request.java
+++ b/jetty-core/jetty-server/src/main/java/org/eclipse/jetty/server/Request.java
@@ -20,22 +20,22 @@
 import java.nio.ByteBuffer;
 import java.nio.charset.Charset;
 import java.security.Principal;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Locale;
 import java.util.concurrent.TimeoutException;
 import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.function.Predicate;
-import java.util.stream.Collectors;
 
 import org.eclipse.jetty.http.CookieCache;
 import org.eclipse.jetty.http.HttpCookie;
-import org.eclipse.jetty.http.HttpField;
 import org.eclipse.jetty.http.HttpFields;
 import org.eclipse.jetty.http.HttpHeader;
 import org.eclipse.jetty.http.HttpScheme;
 import org.eclipse.jetty.http.HttpURI;
 import org.eclipse.jetty.http.MetaData;
+import org.eclipse.jetty.http.MimeTypes;
 import org.eclipse.jetty.http.Trailers;
 import org.eclipse.jetty.io.Content;
 import org.eclipse.jetty.server.internal.HttpChannelState;
@@ -119,6 +119,7 @@ public interface Request extends Attributes, Content.Source
 {
     String CACHE_ATTRIBUTE = Request.class.getCanonicalName() + ".CookieCache";
     String COOKIE_ATTRIBUTE = Request.class.getCanonicalName() + ".Cookies";
+    List<Locale> DEFAULT_LOCALES = List.of(Locale.getDefault());
 
     /**
      * an ID unique within the lifetime scope of the {@link ConnectionMetaData#getId()}).
@@ -452,26 +453,27 @@ static List<Locale> getLocales(Request request)
     {
         HttpFields fields = request.getHeaders();
         if (fields == null)
-            return List.of(Locale.getDefault());
+            return DEFAULT_LOCALES;
 
         List<String> acceptable = fields.getQualityCSV(HttpHeader.ACCEPT_LANGUAGE);
 
-        // handle no locale
-        if (acceptable.isEmpty())
-            return List.of(Locale.getDefault());
-
-        return acceptable.stream().map(language ->
+        // return sorted list of locals, with known locales in quality order before unknown locales in quality order
+        return switch (acceptable.size())
         {
-            language = HttpField.stripParameters(language);
-            String country = "";
-            int dash = language.indexOf('-');
-            if (dash > -1)
+            case 0 -> DEFAULT_LOCALES;
+            case 1 -> List.of(Locale.forLanguageTag(acceptable.get(0)));
+            default ->
             {
-                country = language.substring(dash + 1).trim();
-                language = language.substring(0, dash).trim();
+                List<Locale> locales = acceptable.stream().map(Locale::forLanguageTag).toList();
+                List<Locale> known = locales.stream().filter(MimeTypes::isKnownLocale).toList();
+                if (known.size() == locales.size())
+                    yield locales; // All locales are known
+                List<Locale> unknown = locales.stream().filter(l -> !MimeTypes.isKnownLocale(l)).toList();
+                locales = new ArrayList<>(known);
+                locales.addAll(unknown);
+                yield locales; // List of known locales before unknown locales
             }
-            return new Locale(language, country);
-        }).collect(Collectors.toList());
+        };
     }
 
     // TODO: consider inline and remove.
diff --git a/jetty-core/jetty-server/src/test/java/org/eclipse/jetty/server/RequestTest.java b/jetty-core/jetty-server/src/test/java/org/eclipse/jetty/server/RequestTest.java
index 08d9f4534022..d81bab9c4e73 100644
--- a/jetty-core/jetty-server/src/test/java/org/eclipse/jetty/server/RequestTest.java
+++ b/jetty-core/jetty-server/src/test/java/org/eclipse/jetty/server/RequestTest.java
@@ -17,7 +17,9 @@
 import java.nio.ByteBuffer;
 import java.util.Arrays;
 import java.util.List;
+import java.util.Locale;
 import java.util.concurrent.TimeUnit;
+import java.util.stream.Stream;
 
 import org.eclipse.jetty.http.HttpCookie;
 import org.eclipse.jetty.http.HttpHeader;
@@ -30,6 +32,9 @@
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
 
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.containsString;
@@ -372,6 +377,39 @@ public boolean handle(org.eclipse.jetty.server.Request request, Response respons
         assertThat(response.getStatus(), is(HttpStatus.OK_200));
     }
 
+    public static Stream<Arguments> localeTests()
+    {
+        return Stream.of(
+            Arguments.of(null, List.of(Locale.getDefault().toLanguageTag()).toString()),
+            Arguments.of("zz", "[zz]"),
+            Arguments.of("en", "[en]"),
+            Arguments.of("en-gb", List.of(Locale.UK.toLanguageTag()).toString()),
+            Arguments.of("en-us", List.of(Locale.US.toLanguageTag()).toString()),
+            Arguments.of("EN-US", List.of(Locale.US.toLanguageTag()).toString()),
+            Arguments.of("en-us,en-gb", List.of(Locale.US.toLanguageTag(), Locale.UK.toLanguageTag()).toString()),
+            Arguments.of("en-us;q=0.5,fr;q=0.0,en-gb;q=1.0", List.of(Locale.UK.toLanguageTag(), Locale.US.toLanguageTag()).toString()),
+            Arguments.of("en-us;q=0.5,zz-yy;q=0.7,en-gb;q=1.0", List.of(Locale.UK.toLanguageTag(), Locale.US.toLanguageTag(), "zz-YY").toString())
+        );
+    }
+
+    @ParameterizedTest
+    @MethodSource("localeTests")
+    public void testAcceptableLocales(String acceptLanguage, String expectedLocales) throws Exception
+    {
+        acceptLanguage = acceptLanguage == null ? "" : (HttpHeader.ACCEPT_LANGUAGE.asString() + ": " + acceptLanguage + "\n");
+        String rawRequest = """
+                GET / HTTP/1.1
+                Host: tester
+                Connection: close
+                %s
+                """.formatted(acceptLanguage);
+
+        HttpTester.Response response = HttpTester.parseResponse(connector.getResponse(rawRequest));
+        assertNotNull(response);
+        assertThat(response.getStatus(), is(HttpStatus.OK_200));
+        assertThat(response.getContent(), containsString("locales=" + expectedLocales));
+    }
+
     private static void checkCookieResult(String containedCookie, String[] notContainedCookies, String response)
     {
         assertNotNull(containedCookie);
diff --git a/jetty-core/jetty-server/src/test/java/org/eclipse/jetty/server/handler/DumpHandler.java b/jetty-core/jetty-server/src/test/java/org/eclipse/jetty/server/handler/DumpHandler.java
index 783b4e763042..8e4037113556 100644
--- a/jetty-core/jetty-server/src/test/java/org/eclipse/jetty/server/handler/DumpHandler.java
+++ b/jetty-core/jetty-server/src/test/java/org/eclipse/jetty/server/handler/DumpHandler.java
@@ -17,6 +17,7 @@
 import java.io.OutputStreamWriter;
 import java.io.Writer;
 import java.nio.charset.StandardCharsets;
+import java.util.Locale;
 
 import org.eclipse.jetty.http.HttpField;
 import org.eclipse.jetty.http.HttpHeader;
@@ -151,6 +152,7 @@ public boolean handle(Request request, Response response, Callback callback) thr
         writer.write("<pre>httpURI.path=" + httpURI.getPath() + "</pre><br/>\n");
         writer.write("<pre>httpURI.query=" + httpURI.getQuery() + "</pre><br/>\n");
         writer.write("<pre>httpURI.pathQuery=" + httpURI.getPathQuery() + "</pre><br/>\n");
+        writer.write("<pre>locales=" + Request.getLocales(request).stream().map(Locale::toLanguageTag).toList() + "</pre><br/>\n");
         writer.write("<pre>pathInContext=" + Request.getPathInContext(request) + "</pre><br/>\n");
         writer.write("<pre>contentType=" + request.getHeaders().get(HttpHeader.CONTENT_TYPE) + "</pre><br/>\n");
         writer.write("<pre>servername=" + Request.getServerName(request) + "</pre><br/>\n");