feat: update MainForm component labels for consistency and localization; enhance i18next support in translation files

frontend/src/utils/i18n/locales/en/en.json

@@ -1,3 +1,12 @@
 {
-    "phone_number": "Telefonnummer"
-}
+    "first-name": "First Name",
+    "last-name": "Last Name",
+    "phone-number": "Phone Number",
+    "tickets": "Tickets",
+    "invoice": "Invoice",
+    "company-name": "Company Name",
+    "street": "Street + House No.",
+    "postal-code": "Postal Code + City",
+    "email": "Email",
+    "submit": "Buy"
+}