fix: validateLanguage keeps the player's language instead of always en_GB (#937)

src/main/java/utils/Validate.java

@@ -5,6 +5,7 @@
 import jakarta.servlet.http.Cookie;
 import jakarta.servlet.http.HttpSession;
 import java.math.BigInteger;
+import java.util.Set;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 
@@ -345,28 +346,31 @@ public static String validateEncryptionKey(String userSalt) {
    * Function that will check if a valid language is set. if not, returns en (English)
    *
    * @param ses Session Language Parameter
-   * @return en by default, or the valid setting found in the submitted lang
+   * @return The supported language code in the session's lang (a picker value such as "es", or a
+   *     locale such as "es_ES"), or en_GB by default
    */
   public static String validateLanguage(HttpSession ses) {
     String result = "en_GB";
-    String lang = new String();
+    String lang = "";
 
-    try {
+    if (ses != null && ses.getAttribute("lang") != null) {
       lang = ses.getAttribute("lang").toString();
-    } catch (NullPointerException e) {
-      lang = "";
     }
-    // log.debug("lang submitted: " + lang);
-    if (lang != null) {
-      if (lang.matches(".[a-z]{2}-[A-Z]{2}$")) {
-        result = lang;
+    // A two-letter language, optionally followed by a region (es, es_ES, pt-BR)
+    if (lang.matches("^[a-z]{2}([_-][A-Za-z]{2})?$")) {
+      String language = lang.substring(0, 2);
+      if (SUPPORTED_LANGUAGES.contains(language)) {
+        result = language;
       }
     }
-    // log.debug("lang set to: " + result);
 
     return result;
   }
 
+  /** The languages offered by translation-select.jsp */
+  private static final Set<String> SUPPORTED_LANGUAGES =
+      Set.of("en", "es", "fr", "pt", "ga", "zh", "hi", "ru");
+
   /**
    * Validates objects received through a function request. Also ensures max length is not too high.
    *

src/test/java/utils/ValidateLanguageTest.java

@@ -0,0 +1,45 @@
+package utils;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.Locale;
+import org.junit.jupiter.api.Test;
+import org.springframework.mock.web.MockHttpSession;
+
+/** Validate.validateLanguage keeps the language the player picked (#937) */
+class ValidateLanguageTest {
+
+  private static String languageFor(Object lang) {
+    MockHttpSession session = new MockHttpSession();
+    session.setAttribute("lang", lang);
+    return Validate.validateLanguage(session);
+  }
+
+  @Test
+  public void pickerLanguages_AreKept() {
+    for (String lang : new String[] {"en", "es", "fr", "pt", "ga", "zh", "hi", "ru"}) {
+      assertEquals(lang, languageFor(lang), lang);
+    }
+  }
+
+  @Test
+  public void languageWithRegion_IsReducedToTheLanguage() {
+    assertEquals("es", languageFor("es_ES"));
+    assertEquals("pt", languageFor("pt-BR"));
+    // translation.jsp stores the request's Locale when no language was picked
+    assertEquals("fr", languageFor(Locale.FRANCE));
+  }
+
+  @Test
+  public void unsupportedOrMalformed_FallsBackToDefault() {
+    for (String lang : new String[] {"de", "xen-GB", "EN", "english", "../es", "es_ES_x", ""}) {
+      assertEquals("en_GB", languageFor(lang), lang);
+    }
+  }
+
+  @Test
+  public void missingLanguage_FallsBackToDefault() {
+    assertEquals("en_GB", Validate.validateLanguage(new MockHttpSession()));
+    assertEquals("en_GB", Validate.validateLanguage(null));
+  }
+}