SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '1646', '1651', '1208', '1203', '1206', 
    '1351', '1484', '1486', '1434', '1441', 
    '1468', '1435', '1438', '1439', '1440', 
    '1453', '1467', '1454', '1436', '1427', 
    '1426', '1425', '1429', '1431', '1433', 
    '1207', '1209'
  ) 
  AND lang_code = 'ru'

Query time 0.00049

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": 27,
      "filtered": 100,
      "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('1646','1651','1208','1203','1206','1351','1484','1486','1434','1441','1468','1435','1438','1439','1440','1453','1467','1454','1436','1427','1426','1425','1429','1431','1433','1207','1209') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
    }
  }
}