SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    19869, 19870, 19871, 19872, 19873, 19874, 
    19875, 19876, 19877, 19878, 19879, 
    19880, 19881, 19882, 19883, 19884
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00068

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 (19869,19870,19871,19872,19873,19874,19875,19876,19877,19878,19879,19880,19881,19882,19883,19884) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}