SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    19661, 19662, 19663, 19664, 19665, 19666, 
    19667, 19668, 19669, 19670, 19671, 
    19672, 19673, 19674, 19675, 19676
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00994

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 (19661,19662,19663,19664,19665,19666,19667,19668,19669,19670,19671,19672,19673,19674,19675,19676) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}