As far as I understand it, both papers would be the same paper except for the language in which case it would simply be a translation of an existing paper. This would be the same as translating a book or any other article and would be ethical unless there were any changes to the content that were not due to language differences. I look forward to reading other people's views on this.
I do not think that you should submit it to a different journal that publishes in Japanese. But you can make an author's translation available. The style manual (APA, Chicago, etc.) you use should address how to do this. In ResearchGate, it would be easy to provide the translation as an additional resource for the publication.
I have done this and it has never been suggested that it is unethical. What you need to do is (a) get written permission from the publisher of the original article, (b) make it completely clear to the editor/publisher of the journal where you wish to publish the translation that it is a translation, and (c) include a footnote in a prominent position on the translation (preferably the first page) stating that the article is a translation and giving the full reference for the original article. Translating an article can have several advantages. Two of the main ones are that (a) it becomes accessible to readers who cannot read or have difficulties reading the language of the original article, and (b) it can help to create technical terminology in the target language of the translation, which can stimulate others to write about the subject in that language.