SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20685, 20686, 20687, 20688, 20689, 20690, 
    20691, 20692, 20693, 20694, 20695, 
    20696, 20697, 20698, 20699, 20700
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00937

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 (20685,20686,20687,20688,20689,20690,20691,20692,20693,20694,20695,20696,20697,20698,20699,20700) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}