Translations: restore key coverage and fix XML validation

Add English fallbacks for 12 missing keys in 41 language files.
Report all missing keys and XML errors before failing validation.

Run four regression tests in CI. All tests and 43 XML files pass.

.github/workflows/xmlvalidate.yml

@@ -7,12 +7,14 @@ on:
       - 'Translations/*'
       - 'src/Common/Language.xml'
       - '.github/workflows/xmlvalidate.*'
+      - 'Tests/test_xmlvalidate.py'
   pull_request:
     branches: [ "master" ]
     paths:
       - 'Translations/*'
       - 'src/Common/Language.xml'
       - '.github/workflows/xmlvalidate.*'
+      - 'Tests/test_xmlvalidate.py'
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
@@ -32,5 +34,7 @@ jobs:
         node-version: 'latest'
     - name: Install fast-xml-parser
       run: npm install fast-xml-parser@4.5.2 -g
+    - name: Test XML validator
+      run: python3 Tests/test_xmlvalidate.py
     - name: Run XML validator script
       run: ${{ github.workspace }}/.github/workflows/xmlvalidate.sh "${{ github.workspace }}"