SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21663, 21664, 21665, 21666, 21667, 21668, 
    21669, 21670, 21671, 21672, 21673, 
    21674, 21675, 21676, 21677, 21678
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00911

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 (21663,21664,21665,21666,21667,21668,21669,21670,21671,21672,21673,21674,21675,21676,21677,21678) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}