SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21839, 21840, 21841, 21842, 21843, 21844, 
    21845, 21846, 21847, 21848, 21849, 
    21850, 21851, 21852, 21853, 21854
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00030

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 (21839,21840,21841,21842,21843,21844,21845,21846,21847,21848,21849,21850,21851,21852,21853,21854) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}