SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21229, 21230, 21231, 21232, 21233, 21234, 
    21235, 21236, 21237, 21238, 21239, 
    21240, 21241, 21242, 21243, 21244
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00128

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 (21229,21230,21231,21232,21233,21234,21235,21236,21237,21238,21239,21240,21241,21242,21243,21244) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}