SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '1208', '1203', '1434', '1456', '1441', 
    '1466', '1464', '1435', '1438', '1439', 
    '1465', '1440', '1453', '1467', '1454', 
    '1436', '1425', '1424', '1429', '1431', 
    '1433', '1207', '1209'
  ) 
  AND lang_code = 'ru'

Query time 0.00035

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_feature_variant_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["variant_id", "lang_code"],
      "rows": 23,
      "filtered": 100,
      "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('1208','1203','1434','1456','1441','1466','1464','1435','1438','1439','1465','1440','1453','1467','1454','1436','1425','1424','1429','1431','1433','1207','1209') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
    }
  }
}