git: Add script to check localized texts occurrences in code (#1511)

This script will be executed on every CI run as part of tests, and will
ensure no unlocalised strings are present in the code

Note that texts without the `_lang` suffix will not be checked, e.g.
96fe608d60/plugins/builtin/source/content/views/view_provider_settings.cpp (L10)

.github/workflows/tests.yml

@@ -61,3 +61,17 @@ jobs:
       run: |
         cd build
         ctest --output-on-failure
+
+  langs:
+    name: 🧪 Langs
+    runs-on: ubuntu-22.04
+
+    steps:
+    - name: 🧰 Checkout
+      uses: actions/checkout@v3
+      with:
+        submodules: recursive
+
+    - name: Check langs
+      run:
+        python3 tests/check_langs.py