fixed compatibility
diff --git a/impl/src/main/java/org/apache/myfaces/config/element/LocaleConfig.java b/impl/src/main/java/org/apache/myfaces/config/element/LocaleConfig.java
index a48697b..a4bf051 100644
--- a/impl/src/main/java/org/apache/myfaces/config/element/LocaleConfig.java
+++ b/impl/src/main/java/org/apache/myfaces/config/element/LocaleConfig.java
@@ -31,4 +31,5 @@
 
     public abstract List<String> getSupportedLocales();
     
+    public abstract void addSupportedLocale(String locale);
 }
diff --git a/impl/src/main/java/org/apache/myfaces/config/impl/FacesConfigUnmarshallerImpl.java b/impl/src/main/java/org/apache/myfaces/config/impl/FacesConfigUnmarshallerImpl.java
index 7a5874a..3fdd128 100755
--- a/impl/src/main/java/org/apache/myfaces/config/impl/FacesConfigUnmarshallerImpl.java
+++ b/impl/src/main/java/org/apache/myfaces/config/impl/FacesConfigUnmarshallerImpl.java
@@ -186,7 +186,7 @@
             {
                 if (!localeConfig.getSupportedLocales().contains(localeConfig.getDefaultLocale()))
                 {
-                    localeConfig.getSupportedLocales().add(localeConfig.getDefaultLocale());
+                    localeConfig.addSupportedLocale(localeConfig.getDefaultLocale());
                 }
             }
         }
diff --git a/impl/src/main/java/org/apache/myfaces/config/impl/element/LocaleConfigImpl.java b/impl/src/main/java/org/apache/myfaces/config/impl/element/LocaleConfigImpl.java
index ed1009c..ce184c9 100755
--- a/impl/src/main/java/org/apache/myfaces/config/impl/element/LocaleConfigImpl.java
+++ b/impl/src/main/java/org/apache/myfaces/config/impl/element/LocaleConfigImpl.java
@@ -20,6 +20,7 @@
 
 import java.io.Serializable;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 
 /**
@@ -35,6 +36,7 @@
         this.defaultLocale = defaultLocale;
     }
 
+    @Override
     public void addSupportedLocale(String locale)
     {
         if (supportedLocales == null)
@@ -55,7 +57,7 @@
     {
         if (supportedLocales == null)
         {
-            supportedLocales = new ArrayList<>();
+            return Collections.emptyList();
         }
         return supportedLocales;
     }