Assuming the content is in Japanese this is probably the expected result though.
For international websites it would be good to follow the users chosen language. But if a website doesn't have a english version(or other alphabet based version), expecting alphabets in a captcha is not very reasonable.
> expecting alphabets in a captcha is not very reasonable.
As an American expat living in a country that doesn't speak English by default... I'd say it's fairly reasonable. Captchas aren't the responsibility of the site owner, the captcha integration docs should explain how to pass the language through to them and should require developers to do so.
For international websites it would be good to follow the users chosen language. But if a website doesn't have a english version(or other alphabet based version), expecting alphabets in a captcha is not very reasonable.