SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21919, 21920, 21921, 21922, 21923, 21924, 
    21925, 21926, 21927, 21928, 21929, 
    21930, 21931, 21932, 21933, 21934
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00080

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 (21919,21920,21921,21922,21923,21924,21925,21926,21927,21928,21929,21930,21931,21932,21933,21934) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}