diff --git a/services/src/main/java/org/keycloak/theme/FolderTheme.java b/services/src/main/java/org/keycloak/theme/FolderTheme.java
index 18b8e128be..b06631b0d7 100644
--- a/services/src/main/java/org/keycloak/theme/FolderTheme.java
+++ b/services/src/main/java/org/keycloak/theme/FolderTheme.java
@@ -107,7 +107,7 @@ public class FolderTheme implements Theme {
         return getMessages("messages", locale);
     }
 
-    private static final Pattern LEGAL_LOCALE = Pattern.compile("[a-zA-Z0-9-_]*");
+    private static final Pattern LEGAL_LOCALE = Pattern.compile("[a-zA-Z0-9-_#]*");
 
     @Override
     public Properties getMessages(String baseBundlename, Locale locale) throws IOException {
diff --git a/services/src/test/java/org/keycloak/theme/FolderThemeTest.java b/services/src/test/java/org/keycloak/theme/FolderThemeTest.java
new file mode 100644
index 0000000000..a1bba9e923
--- /dev/null
+++ b/services/src/test/java/org/keycloak/theme/FolderThemeTest.java
@@ -0,0 +1,42 @@
+/*
+ * Copyright 2016 Red Hat, Inc. and/or its affiliates
+ * and other contributors as indicated by the @author tags.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.keycloak.theme;
+
+import static org.junit.Assert.*;
+import static org.keycloak.theme.Theme.Type.LOGIN;
+
+import org.junit.Test;
+
+import java.io.File;
+import java.util.Locale;
+import java.util.Set;
+
+public class FolderThemeTest {
+
+    private final Set<String> someValidISOLocaleCodes = Set.of("de", "de-DE", "de-x-informal");
+
+    @Test
+    public void testLocaleWithExtension() throws Exception {
+        FolderTheme theme = new FolderTheme(new File("."), "name", LOGIN);
+
+        for (String localeCode : someValidISOLocaleCodes) {
+            Locale locale = Locale.forLanguageTag(localeCode);
+            assertNotNull(theme.getMessages("base", locale));
+        }
+    }
+}