SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21711, 21712, 21713, 21714, 21715, 21716, 
    21717, 21718, 21719, 21720, 21721, 
    21722, 21723, 21724, 21725, 21726
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00037

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "product_id", "idx_product_lang"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (21711,21712,21713,21714,21715,21716,21717,21718,21719,21720,21721,21722,21723,21724,21725,21726) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}