SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20925, 20926, 20927, 20928, 20929, 20930, 
    20931, 20932, 20933, 20934, 20935, 
    20936, 20937, 20938, 20939, 20940
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00948

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 (20925,20926,20927,20928,20929,20930,20931,20932,20933,20934,20935,20936,20937,20938,20939,20940) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}