Refactor code and improve internationalization support

- Updated import statements to remove invisible characters.
- Standardized comments to use a consistent hyphen format.
- Adjusted username validation error messages for consistency.
- Enhanced email sending functions to include language support.
- Added email internationalization configuration for dynamic translations.
- Updated calendar and federation routes to include language in user queries.
- Improved user feedback messages in German and English for clarity.

server/routes/calendar.js

@@ -1,4 +1,4 @@
-import { Router } from 'express';
+import { Router } from 'express';
 import crypto from 'crypto';
 import { getDb } from '../config/database.js';
 import { authenticateToken } from '../middleware/auth.js';
@@ -289,7 +289,7 @@ router.post('/events/:id/share', authenticateToken, async (req, res) => {
     );
 
     // Send notification email (fire-and-forget)
-    const targetUser = await db.get('SELECT name, display_name, email FROM users WHERE id = ?', [user_id]);
+    const targetUser = await db.get('SELECT name, display_name, email, language FROM users WHERE id = ?', [user_id]);
     if (targetUser?.email) {
       const appUrl = process.env.APP_URL || `${req.protocol}://${req.get('host')}`;
       const inboxUrl = `${appUrl}/federation/inbox`;
@@ -301,7 +301,7 @@ router.post('/events/:id/share', authenticateToken, async (req, res) => {
         (targetUser.display_name && targetUser.display_name !== '') ? targetUser.display_name : targetUser.name,
         fromDisplay,
         event.title, event.start_time, event.end_time, event.description,
-        inboxUrl, appName
+        inboxUrl, appName, targetUser.language || 'en'
       ).catch(mailErr => {
         log.server.warn('Calendar local share mail failed (non-fatal):', mailErr.message);
       });
@@ -423,7 +423,7 @@ router.delete('/local-invitations/:id', authenticateToken, async (req, res) => {
     if (inv.status === 'pending') {
       await db.run("UPDATE calendar_local_invitations SET status = 'declined' WHERE id = ?", [inv.id]);
     } else {
-      // Accepted/declined – remove the share too if it was accepted
+      // Accepted/declined - remove the share too if it was accepted
       if (inv.status === 'accepted') {
         await db.run('DELETE FROM calendar_event_shares WHERE event_id = ? AND user_id = ?', [inv.event_id, req.user.id]);
       }
@@ -587,7 +587,7 @@ router.post(['/receive-event', '/calendar-event'], calendarFederationLimiter, as
     // Find local user
     const { username } = parseAddress(to_user);
     const db = getDb();
-    const targetUser = await db.get('SELECT id, name, email FROM users WHERE LOWER(name) = LOWER(?)', [username]);
+    const targetUser = await db.get('SELECT id, name, email, language FROM users WHERE LOWER(name) = LOWER(?)', [username]);
     if (!targetUser) return res.status(404).json({ error: 'User not found on this instance' });
 
     // Check duplicate (already in invitations or already accepted into calendar)
@@ -619,7 +619,7 @@ router.post(['/receive-event', '/calendar-event'], calendarFederationLimiter, as
       sendCalendarInviteEmail(
         targetUser.email, targetUser.name, from_user,
         title, start_time, end_time, description || null,
-        inboxUrl, appName
+        inboxUrl, appName, targetUser.language || 'en'
       ).catch(mailErr => {
         log.server.warn('Calendar invite mail failed (non-fatal):', mailErr.message);
       });