SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    18429, 18430, 18431, 18432, 18433, 18434, 
    18435, 18436, 18437, 18438, 18439, 
    18440, 18441, 18442, 18443, 18444
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00045

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 (18429,18430,18431,18432,18433,18434,18435,18436,18437,18438,18439,18440,18441,18442,18443,18444) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}