Hi, unfortunatley as far as I can tell there is no supported way of passing the language of the browser into the iframe for it to know to translate the content inside the iframe. The only workaround is to just change all the labels and titles that you can control in the source dashboard to Japanese. Obviously this doesn't impact the global filters title. If you needed this to be multilingual then you could potentially try to creating a dashboard for each language and then change the embedded url based on the languge setting of the browser. But this would be a large manual effort. If this is a key requirement for you, I'd suggest you contact your account team to raise a feature request.

Thanks,

Emma