SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20941, 20942, 20943, 20944, 20945, 20946, 
    20947, 20948, 20949, 20950, 20951, 
    20952, 20953, 20954, 20955, 20956
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00143

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 (20941,20942,20943,20944,20945,20946,20947,20948,20949,20950,20951,20952,20953,20954,20955,20956) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}