Fix spelling in the translations and their English source strings

- "Go Back" in not_found.html did not match the "Go back" key and was
  never translated. "Domain must be given" was missing in de.json.
- English: successful, deactivate, a user, log in, URL, Re-enter
  password. The keys in de.json are renamed accordingly.
- German: "Wird" instead of "Word", commas before "da", "E-Mail" and
  "E-Mail-Adresse" instead of "Email", "angegeben werden" instead of
  "gegeben sein", "ungültig" instead of "valide", "selbst" instead of
  "selber", missing periods.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

max/templates/not_found.html

@@ -8,7 +8,7 @@
         <div class="pure-u-1-5">
             <a href="/" class="top-row-button pure-button">
                 <i class="fa fa-arrow-left"></i>
-                {{ t("Go Back") }}
+                {{ t("Go back") }}
             </a>
         </div>
     </div>