SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20461, 20462, 20463, 20464, 20465, 20466, 
    20467, 20468, 20469, 20470, 20471, 
    20472, 20473, 20474, 20475, 20476
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00197

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 (20461,20462,20463,20464,20465,20466,20467,20468,20469,20470,20471,20472,20473,20474,20475,20476) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}