Check avaiable help lanugages when opening specific help page

CHANGELOG.md

@@ -58,6 +58,7 @@ We refer to [GitHub issues](https://github.com/JabRef/jabref/issues) by using `#
 - Fixed InvalidBackgroundColor flickering with Ctrl-s and File > Save database
 - Fixed loop when pulling changes (shared database) when current selected field has changed
 - Fixed [#1958](https://github.com/JabRef/jabref/issues/1958): Verbatim fields are no longer checked for HTML encoded characters by integrity checks
+- Fixed [#1937](https://github.com/JabRef/jabref/issues/1937): If no help page for the current chosen language exists, the english help page will be shown
 
 ### Removed
 - The non-supported feature of being able to define file directories for any extension is removed. Still, it should work for older databases using the legacy `ps` and `pdf` fields, although we strongly encourage using the `file` field. 

src/main/java/net/sf/jabref/gui/help/HelpAction.java

@@ -6,6 +6,10 @@
 import java.awt.event.ActionEvent;
 import java.awt.event.MouseAdapter;
 import java.awt.event.MouseEvent;
+import java.util.HashSet;
+import java.util.Set;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
 import javax.swing.Action;
 import javax.swing.Icon;
@@ -27,6 +31,12 @@
  */
 public class HelpAction extends MnemonicAwareAction {
 
+    /**
+     * New languages of the help have to be added here
+     */
+    private static final Set<String> avaiableLangFiles = Stream.of("en", "de", "fr", "in", "ja")
+            .collect(Collectors.toCollection(HashSet::new));
+
     private HelpFile helpPage;
 
 
@@ -67,6 +77,7 @@ public JLabel getHelpLabel(String labelText) {
         helpLabel.setForeground(Color.BLUE);
         helpLabel.setCursor(new Cursor(Cursor.HAND_CURSOR));
         helpLabel.addMouseListener(new MouseAdapter() {
+
             @Override
             public void mouseClicked(MouseEvent e) {
                 openHelpPage();
@@ -85,7 +96,16 @@ public void actionPerformed(ActionEvent e) {
     }
 
     private void openHelpPage() {
-        String url = "https://help.jabref.org/" + Globals.prefs.get(JabRefPreferences.LANGUAGE) + "/" + helpPage.getPageName();
-        JabRefDesktop.openBrowserShowPopup(url);
+        String lang = Globals.prefs.get(JabRefPreferences.LANGUAGE);
+        StringBuilder sb = new StringBuilder("https://help.jabref.org/");
+
+        if (avaiableLangFiles.contains(lang)) {
+            sb.append(lang);
+            sb.append("/");
+        } else {
+            sb.append("en/");
+        }
+        sb.append(helpPage.getPageName());
+        JabRefDesktop.openBrowserShowPopup(sb.toString());
     }
 }