SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    19645, 19646, 19647, 19648, 19649, 19650, 
    19651, 19652, 19653, 19654, 19655, 
    19656, 19657, 19658, 19659, 19660
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

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 (19645,19646,19647,19648,19649,19650,19651,19652,19653,19654,19655,19656,19657,19658,19659,19660) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}