SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20637, 20638, 20639, 20640, 20641, 20642, 
    20643, 20644, 20645, 20646, 20647, 
    20648, 20649, 20650, 20651, 20652
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00039

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 (20637,20638,20639,20640,20641,20642,20643,20644,20645,20646,20647,20648,20649,20650,20651,20652) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}