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/config/emaili18n.js

@@ -0,0 +1,52 @@
+import { createRequire } from 'module';
+import { fileURLToPath } from 'url';
+import path from 'path';
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+
+const require = createRequire(import.meta.url);
+const cache = {};
+
+function load(lang) {
+  if (cache[lang]) return cache[lang];
+  try {
+    cache[lang] = require(path.resolve(__dirname, '../../src/i18n', `${lang}.json`));
+    return cache[lang];
+  } catch {
+    if (lang !== 'en') return load('en');
+    cache[lang] = {};
+    return cache[lang];
+  }
+}
+
+/**
+ * Translate a dot-separated key for the given language.
+ * Interpolates {placeholder} tokens from params.
+ * Unresolved tokens are left as-is so callers can do HTML substitution afterwards.
+ *
+ * @param {string} lang    Language code, e.g. 'en', 'de'
+ * @param {string} keyPath Dot-separated key, e.g. 'email.verify.subject'
+ * @param {Record<string,string>} [params]  Values to interpolate
+ * @returns {string}
+ */
+export function t(lang, keyPath, params = {}) {
+  const keys = keyPath.split('.');
+
+  function resolve(dict) {
+    let val = dict;
+    for (const k of keys) {
+      val = val?.[k];
+    }
+    return typeof val === 'string' ? val : undefined;
+  }
+
+  let value = resolve(load(lang));
+  // Fallback to English
+  if (value === undefined) value = resolve(load('en'));
+  if (value === undefined) return keyPath;
+
+  return value.replace(/\{(\w+)\}/g, (match, k) =>
+    params[k] !== undefined ? String(params[k]) : match
+  );
+}