Store refdoc preferred language

See gh-21778

src/docs/asciidoc/js/switch-language.js

 function globalSwitch() {
+
+    var SPRING_LANGUAGES = ["Java", "Kotlin"];
+    var preferredLanguage = initPreferredLanguage();
+
+    function initPreferredLanguage() {
+        var lang = window.localStorage.getItem("preferred-spring-language");
+        if (SPRING_LANGUAGES.indexOf(lang) === -1) {
+            window.localStorage.setItem("preferred-spring-language", SPRING_LANGUAGES[0]);
+            lang = SPRING_LANGUAGES[0];
+        }
+        return lang;
+    }
+
+    function switchItem(text, index) {
+        if (SPRING_LANGUAGES.indexOf(text) !== -1) {
+            window.localStorage.setItem("preferred-spring-language", text);
+        }
+        $(".switch--item").filter(function() { return ($(this).text() === text) }).each(function() {
+            $(this).addClass('selected');
+            $(this).siblings().removeClass('selected');
+            var selectedContent = $(this).parent().siblings(".content").eq(index)
+            selectedContent.removeClass('hidden');
+            selectedContent.siblings().addClass('hidden');
+        });
+    }
+
     $('.switch--item').each(function() {
         $(this).off('click');
         $(this).on('click', function() {
-            selectedText = $(this).text()
-            selectedIndex = $(this).index()
-            $(".switch--item").filter(function() { return ($(this).text() === selectedText) }).each(function() {
-                $(this).addClass('selected');
-                $(this).siblings().removeClass('selected');
-                selectedContent = $(this).parent().siblings(".content").eq(selectedIndex)
-                selectedContent.removeClass('hidden');
-                selectedContent.siblings().addClass('hidden');
-            });
+            var selectedText = $(this).text()
+            var selectedIndex = $(this).index()
+            switchItem(selectedText, selectedIndex);
         });
     });
+
+    var languageIndex = SPRING_LANGUAGES.indexOf(preferredLanguage);
+    if (languageIndex != 0) {
+        switchItem(preferredLanguage, languageIndex);
+    }
 }
 
 $(globalSwitch);
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