SELECT 
  pd.product_id as product_id, 
  pd.product as product, 
  IF(
    shared_descr.product_id IS NOT NULL, 
    shared_descr.product, pd.product
  ) as product 
FROM 
  cscart_product_descriptions pd 
  LEFT JOIN cscart_ult_product_descriptions shared_descr ON shared_descr.product_id = pd.product_id 
  AND shared_descr.company_id = 2 
  AND shared_descr.lang_code = 'ja' 
WHERE 
  1 
  AND pd.product_id = 277 
  AND pd.lang_code = 'ja'

Query time 0.00028

JSON explain

{
  "query_block": {
    "select_id": 1,
    "const_condition": "1",
    "table": {
      "table_name": "pd",
      "access_type": "const",
      "possible_keys": ["PRIMARY", "product_id"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "shared_descr",
      "access_type": "const",
      "possible_keys": ["PRIMARY", "product_id", "company_id"],
      "key": "PRIMARY",
      "key_length": "13",
      "used_key_parts": ["product_id", "lang_code", "company_id"],
      "ref": ["const", "const", "const"],
      "rows": 1,
      "filtered": 100
    }
  }
}