SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21871, 21872, 21873, 21874, 21875, 21876, 
    21877, 21878, 21879, 21880, 21881, 
    21882, 21883, 21884, 21885, 21886
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00031

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 (21871,21872,21873,21874,21875,21876,21877,21878,21879,21880,21881,21882,21883,21884,21885,21886) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}