SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20877, 20878, 20879, 20880, 20881, 20882, 
    20883, 20884, 20885, 20886, 20887, 
    20888, 20889, 20890, 20891, 20892
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00039

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 (20877,20878,20879,20880,20881,20882,20883,20884,20885,20886,20887,20888,20889,20890,20891,20892) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}