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/index.js

@@ -1,4 +1,4 @@
-import 'dotenv/config';
+import 'dotenv/config';
 import express from 'express';
 import cors from 'cors';
 import path from 'path';
@@ -22,7 +22,7 @@ const __dirname = path.dirname(__filename);
 const app = express();
 const PORT = process.env.PORT || 3001;
 
-// Trust proxy – configurable via TRUST_PROXY env var (default: 1 = one local reverse proxy)
+// Trust proxy - configurable via TRUST_PROXY env var (default: 1 = one local reverse proxy)
 // Use a number to trust that many hops, or a string like 'loopback' / an IP/CIDR.
 const rawTrustProxy = process.env.TRUST_PROXY ?? 'loopback';
 const trustProxy = /^\d+$/.test(rawTrustProxy) ? parseInt(rawTrustProxy, 10) : rawTrustProxy;