SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21375, 21376, 21377, 21378, 21379, 21380, 
    21381, 21382, 21383, 21384, 21385, 
    21386, 21387, 21388, 21389, 21390
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00115

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 (21375,21376,21377,21378,21379,21380,21381,21382,21383,21384,21385,21386,21387,21388,21389,21390) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}