SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21487, 21488, 21489, 21490, 21491, 21492, 
    21493, 21494, 21495, 21496, 21497, 
    21498, 21499, 21500, 21501, 21502
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00037

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 (21487,21488,21489,21490,21491,21492,21493,21494,21495,21496,21497,21498,21499,21500,21501,21502) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}