Fix DefaultI18N to prevent fallback to JVM default locale (#26)

---------
Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: slachiewicz <[email protected]>

Author: Copilot

src/main/java/org/codehaus/plexus/i18n/DefaultI18N.java

@@ -298,7 +298,10 @@ private synchronized ResourceBundle cacheBundle(String bundleName, Locale locale
         if (rb == null) {
             bundlesByLocale = (bundlesByLocale == null ? new HashMap<>(3) : new HashMap<>(bundlesByLocale));
             try {
-                rb = ResourceBundle.getBundle(bundleName, locale);
+                rb = ResourceBundle.getBundle(
+                        bundleName,
+                        locale,
+                        ResourceBundle.Control.getNoFallbackControl(ResourceBundle.Control.FORMAT_DEFAULT));
             } catch (MissingResourceException e) {
                 rb = findBundleByLocale(bundleName, locale, bundlesByLocale);
                 if (rb == null) {
@@ -371,7 +374,10 @@ private ResourceBundle findBundleByLocale(
      */
     private ResourceBundle getBundleIgnoreException(String bundleName, Locale locale) {
         try {
-            return ResourceBundle.getBundle(bundleName, locale);
+            return ResourceBundle.getBundle(
+                    bundleName,
+                    locale,
+                    ResourceBundle.Control.getNoFallbackControl(ResourceBundle.Control.FORMAT_DEFAULT));
         } catch (MissingResourceException ignored) {
             return null;
         }

src/test/java/org/codehaus/plexus/i18n/DefaultI18NTest.java

@@ -110,7 +110,7 @@ public void testLocalization() {
         Locale.setDefault(Locale.FRENCH);
         try {
             String s7 = i18n.getString("org.codehaus.plexus.i18n.i18n", Locale.ENGLISH, "key1");
-            assertEquals("[fr] value1", s7, "Not picking up new default locale: fr");
+            assertEquals("[] value1", s7, "Should not fall back to default locale, use root bundle instead");
 
             String s8 = i18n.getString("org.codehaus.plexus.i18n.i18n", Locale.ITALIAN, "key1");
             assertEquals("[it] value1", s8, "Unable to retrieve localized properties for locale: it");
@@ -141,4 +141,25 @@ public void testLocalizedMessagesWithNonStandardLocale() {
         String s0 = i18n.getString("name", new Locale("xx"));
         assertEquals("plexus", s0);
     }
+
+    @Test
+    public void testNoFallbackToDefaultLocale() {
+        // Save the current default locale
+        Locale oldDefault = Locale.getDefault();
+
+        try {
+            // Set default locale to German
+            Locale.setDefault(Locale.GERMAN);
+
+            // Request a locale that doesn't have a bundle (Hebrew - Israel)
+            // Expected: should get the root bundle with "[] value1"
+            // Bug: currently gets the German bundle with "[de] value1"
+            String result = i18n.getString("org.codehaus.plexus.i18n.i18n", new Locale("iw", "IL"), "key1");
+
+            assertEquals("[] value1", result, "Should get root bundle, not default locale bundle");
+        } finally {
+            // Restore the original default locale
+            Locale.setDefault(oldDefault);
+        }
+    }
 }

src/test/resources/org/codehaus/plexus/i18n/i18n_de.properties

@@ -0,0 +1,4 @@
+key1 = [de] value1
+thanks.message = Danke {0}!
+thanks.message1 = Danke {0} {1}!
+thanks.message2 = Danke {0} {1}!