feat(locale): the panel offers only the languages it is translated into

The language choice on the Settings page listed every language Django
knows, 99 of them. Six catalogs (Czech under the wrong code, German,
Spanish, French, Russian, Ukrainian) translated none of the panel's
messages, so choosing one of them, like most of the others, showed
English. The Turkish catalog lacked the four newest messages.

The panel now offers English and Turkish (LANGUAGES). The empty
catalogs and the English one, which has nothing to translate, are
removed. Turkish translates every message again. CI checks that the
offered catalogs translate every message and that their compiled
files are up to date; the README says how to update them.

.github/workflows/test.yml

@@ -71,7 +71,8 @@ jobs:
             libldap2-dev \
             libsasl2-dev \
             pkg-config \
-            gcc
+            gcc \
+            gettext
 
       - name: Install Python Dependencies
         run: |
@@ -90,6 +91,19 @@ jobs:
       - name: Run Django Migrations
         run: python manage.py migrate --noinput
 
+      # Each catalog of LANGUAGES translates every message, and its compiled
+      # .mo is the .po's: a changed text needs its translations
+      - name: Check Translation Catalogs
+        if: matrix.python-version == '3.12' && matrix.database == 'sqlite'
+        run: |
+          python manage.py makemessages -l tr --no-obsolete --ignore=staticfiles -v0
+          for dir in locale/tr/LC_MESSAGES; do
+            msgfmt --check -o /tmp/django.mo "$dir/django.po"
+            missing=$(msgattrib --untranslated "$dir/django.po"; msgattrib --only-fuzzy "$dir/django.po")
+            if [ -n "$missing" ]; then echo "$missing"; echo "::error file=$dir/django.po::untranslated or fuzzy messages"; exit 1; fi
+            cmp <(msgunfmt "$dir/django.mo") <(msgunfmt /tmp/django.mo) || { echo "::error file=$dir/django.mo::not compiled from django.po (compilemessages)"; exit 1; }
+          done
+
       # Includes the vrtManager tests; fails below the coverage floor in pyproject.toml
       - name: Run Test Suite
         run: |