SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    19741, 19742, 19743, 19744, 19745, 19746, 
    19747, 19748, 19749, 19750, 19751, 
    19752, 19753, 19754, 19755, 19756
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00034

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 (19741,19742,19743,19744,19745,19746,19747,19748,19749,19750,19751,19752,19753,19754,19755,19756) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}