SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    10656, 10657, 10658, 10659, 10660, 10661, 
    10662, 10663, 10664, 10671, 10673, 
    10675, 10676, 10677, 10678, 10679
  ) 
  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 (10656,10657,10658,10659,10660,10661,10662,10663,10664,10671,10673,10675,10676,10677,10678,10679) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}