feat: add SplitLayoutI18n for customizing split-layout separator label (#9694)

## Description

Part of vaadin/web-components#951

## Type of change

- Feature

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: web-padawan

vaadin-split-layout-flow-parent/vaadin-split-layout-flow/src/main/java/com/vaadin/flow/component/splitlayout/SplitLayout.java

@@ -33,6 +33,7 @@
 import com.vaadin.flow.component.html.Div;
 import com.vaadin.flow.component.shared.HasThemeVariant;
 import com.vaadin.flow.component.shared.SlotUtils;
+import com.vaadin.flow.internal.JacksonUtils;
 import com.vaadin.flow.internal.StateTree;
 import com.vaadin.flow.shared.Registration;
 
@@ -54,6 +55,7 @@ public class SplitLayout extends Component
     private Component secondaryComponent;
     private StateTree.ExecutionRegistration updateStylesRegistration;
     private Double splitterPosition;
+    private SplitLayoutI18n i18n;
 
     /**
      * numeration of all available orientation for VaadinSplitLayout component
@@ -428,4 +430,37 @@ private Double calcNewSplitterPosition(String primaryFlexBasis,
     private double round(double value) {
         return Math.round(value * 100.0) / 100.0;
     }
+
+    /**
+     * Gets the internationalization object previously set for this component.
+     * <p>
+     * NOTE: Updating the instance that is returned from this method will not
+     * update the component if not set again using
+     * {@link #setI18n(SplitLayoutI18n)}.
+     *
+     * @return the i18n object or {@code null} if no i18n object has been set
+     * @since 25.3
+     */
+    public SplitLayoutI18n getI18n() {
+        return i18n;
+    }
+
+    /**
+     * Sets the internationalization object for this component. It enables you
+     * to customize and translate the accessible labels used in the split
+     * layout.
+     * <p>
+     * Note: updating the object properties after setting the i18n will not
+     * update the component. To make the changes effective, you need to set the
+     * updated object again.
+     *
+     * @param i18n
+     *            the i18n object, not {@code null}
+     * @since 25.3
+     */
+    public void setI18n(SplitLayoutI18n i18n) {
+        this.i18n = Objects.requireNonNull(i18n,
+                "The i18n object should not be null");
+        getElement().setPropertyJson("i18n", JacksonUtils.beanToJson(i18n));
+    }
 }

vaadin-split-layout-flow-parent/vaadin-split-layout-flow/src/main/java/com/vaadin/flow/component/splitlayout/SplitLayoutI18n.java

@@ -0,0 +1,61 @@
+/*
+ * Copyright 2000-2026 Vaadin Ltd.
+ *
+ * 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 com.vaadin.flow.component.splitlayout;
+
+import java.io.Serializable;
+
+import com.fasterxml.jackson.annotation.JsonInclude;
+
+/**
+ * The internationalization properties for {@link SplitLayout}. This can be used
+ * to customize and translate the accessible labels used in the split layout
+ * component.
+ *
+ * @see SplitLayout#setI18n(SplitLayoutI18n)
+ *
+ * @author Vaadin Ltd.
+ * @since 25.3
+ */
+@JsonInclude(JsonInclude.Include.NON_NULL)
+public class SplitLayoutI18n implements Serializable {
+    private String separator;
+
+    /**
+     * Gets the accessible label for the splitter that separates the two content
+     * areas.
+     *
+     * @return the accessible label for the splitter
+     */
+    public String getSeparator() {
+        return separator;
+    }
+
+    /**
+     * Sets the accessible label for the splitter that separates the two content
+     * areas.
+     * <p>
+     * This label is used as the {@code aria-label} of the splitter, announced
+     * by screen readers when the splitter is focused.
+     *
+     * @param separator
+     *            the accessible label for the splitter
+     * @return this instance for method chaining
+     */
+    public SplitLayoutI18n setSeparator(String separator) {
+        this.separator = separator;
+        return this;
+    }
+}

vaadin-split-layout-flow-parent/vaadin-split-layout-flow/src/test/java/com/vaadin/flow/component/splitlayout/tests/SplitLayoutI18nTest.java

@@ -0,0 +1,71 @@
+/*
+ * Copyright 2000-2026 Vaadin Ltd.
+ *
+ * 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 com.vaadin.flow.component.splitlayout.tests;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import com.vaadin.flow.component.splitlayout.SplitLayout;
+import com.vaadin.flow.component.splitlayout.SplitLayoutI18n;
+
+class SplitLayoutI18nTest {
+
+    private SplitLayout splitLayout;
+    private SplitLayoutI18n i18n;
+
+    @BeforeEach
+    void setup() {
+        splitLayout = new SplitLayout();
+        i18n = new SplitLayoutI18n();
+    }
+
+    @Test
+    void getI18n_returnsNull() {
+        Assertions.assertNull(splitLayout.getI18n());
+    }
+
+    @Test
+    void setI18n_getI18n() {
+        splitLayout.setI18n(i18n);
+        Assertions.assertSame(i18n, splitLayout.getI18n());
+    }
+
+    @Test
+    void setI18n_null_throws() {
+        Assertions.assertThrows(NullPointerException.class,
+                () -> splitLayout.setI18n(null));
+    }
+
+    @Test
+    void setSeparator_getSeparator() {
+        Assertions.assertNull(i18n.getSeparator());
+        i18n.setSeparator("Resize separator");
+        Assertions.assertEquals("Resize separator", i18n.getSeparator());
+    }
+
+    @Test
+    void separatorSetter_returnsI18n() {
+        Assertions.assertSame(i18n, i18n.setSeparator("foo"));
+    }
+
+    @Test
+    void setI18n_propertySet() {
+        splitLayout.setI18n(i18n.setSeparator("Resize separator"));
+        Assertions.assertEquals("{\"separator\":\"Resize separator\"}",
+                splitLayout.getElement().getProperty("i18n"));
+    }
+}