Clean up i18n: translate 404 page Chinese text and remove placeholder German locale

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cypress/e2e/notFound404.cy.js

@@ -15,13 +15,13 @@ describe('404 Not Found Page', () => {
     loginWithFixtures();
     cy.visit('/this-page-does-not-exist');
     cy.contains('404').should('be.visible');
-    cy.contains('您想找的頁面不存在').should('be.visible');
+    cy.contains('The page you are looking for does not exist.').should('be.visible');
   });
 
   it('has a link back to Files page', () => {
     loginWithFixtures();
     cy.visit('/some/random/path');
-    cy.contains('a', '回到 Files 頁')
+    cy.contains('a', 'Go to Files')
       .should('be.visible')
       .should('have.attr', 'href', '/files');
   });