SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20861, 20862, 20863, 20864, 20865, 20866, 
    20867, 20868, 20869, 20870, 20871, 
    20872, 20873, 20874, 20875, 20876
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00033

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 (20861,20862,20863,20864,20865,20866,20867,20868,20869,20870,20871,20872,20873,20874,20875,20876) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}