能用prusa切片和预览了，添加了缺失的翻译

Co-authored-by: Copilot <copilot@github.com>

app/templates/base.html

@@ -65,6 +65,7 @@
                     <ul class="dropdown-menu dropdown-menu-end shadow" aria-labelledby="langDropdown">
                         <li><a class="dropdown-item {% if request.cookies.get('lang') == 'en' %}active{% endif %}" href="{{ url_for('main.set_language', lang='en') }}"><i class="bi bi-translate me-2"></i>English</a></li>
                         <li><a class="dropdown-item {% if request.cookies.get('lang') == 'zh-cn' %}active{% endif %}" href="{{ url_for('main.set_language', lang='zh-cn') }}"><i class="bi bi-translate me-2"></i>中文 (简体)</a></li>
+                        <li><a class="dropdown-item {% if request.cookies.get('lang') == 'de' %}active{% endif %}" href="{{ url_for('main.set_language', lang='de') }}"><i class="bi bi-translate me-2"></i>Deutsch</a></li>
                     </ul>
                 </div>