SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21887, 21888, 21889, 21890, 21891, 21892, 
    21893, 21894, 21895, 21896, 21897, 
    21898, 21899, 21900, 21901, 21902
  ) 
  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 (21887,21888,21889,21890,21891,21892,21893,21894,21895,21896,21897,21898,21899,21900,21901,21902) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}