Restore hardcoded region lists, sourced from iso_3166-1_alpha-2

Replace fixture-based lookup with the same hardcoded SLAVIC_REGIONS /
SINOPHONE_REGIONS sets defined in cterdam/iso_3166-1_alpha-2
test/test_invariants.py, which is the authoritative source per the README.

test/test_invariants.py

@@ -24,12 +24,24 @@ import pytest
 from collections import defaultdict
 from datasets import load_dataset
 
+# Copied from cterdam/iso_3166-1_alpha-2 test/test_invariants.py — the
+# authoritative definition of which regions use which primary language.
+SLAVIC_REGIONS = {
+    'RU', 'UA', 'BY', 'KZ', 'UZ', 'TJ', 'KG', 'TM', 'GE', 'AZ', 'AM', 'MD',
+    'LT', 'LV', 'EE', 'RS', 'ME', 'BA', 'HR', 'SI', 'SK', 'CZ', 'PL', 'BG', 'MK', 'FI'
+}
 
-@pytest.fixture(scope="session")
-def region_lang():
-    """Load region→lang mapping from cterdam/iso_3166-1_alpha-2 (authoritative)."""
-    ds = load_dataset('cterdam/iso_3166-1_alpha-2', split='train')
-    return {r['code']: r['lang'] for r in ds if r.get('lang')}
+SINOPHONE_REGIONS = {
+    'CN', 'TW', 'HK', 'MO', 'JP', 'KR', 'KP', 'MN', 'SG', 'VN', 'LA', 'TH', 'MM', 'KH', 'MY', 'BN'
+}
+
+
+def determine_expected_lang(region_code: str) -> str:
+    if region_code in SINOPHONE_REGIONS:
+        return 'zh'
+    if region_code in SLAVIC_REGIONS:
+        return 'ru'
+    return 'en'
 
 
 @pytest.fixture(scope="session")
@@ -66,14 +78,14 @@ def test_name_matches_lang_field(dataset):
     assert mismatches == [], f"Name/lang field mismatches: {mismatches}"
 
 
-def test_lang_matches_region(dataset, region_lang):
-    """lang field must match cterdam/iso_3166-1_alpha-2 for that region."""
+def test_lang_matches_region(dataset):
+    """lang field must match the region's language rule."""
     mismatches = []
 
     for i, entry in enumerate(dataset):
         region = entry.get('region') or ''
         lang = entry.get('lang')
-        expected = region_lang.get(region, 'en')
+        expected = determine_expected_lang(region)
 
         if lang != expected:
             mismatches.append((i, entry.get('code'), region, lang, expected))