Hugo allows you to make your website multilingual. Check Multilingual mode for the details.
If you want to set your default language as English, and add Japanese as a secondary language, it can be done by adding
DefaultContentLanguage = "en"
defaultContentLanguageInSubdir = true
and
[Languages]
[Languages.en]
weight = 1
title = "THE TITLE OF YOUR SITE"
[Languages.en.params]
Language_name = "English"
subtitle = "THE SUBTITLE OF YOUR SITE"
...
[Languages.ja]
weight = 2
title = "ウェブサイトのタイトル"
[Languages.ja.params]
Language_name = "日本語"
subtitle = "ウェブサイトのサブタイトル"
...
to the config.toml. For the menus, you can add identical entries for the English version and the Japanese version as
[[Languages.en.menu.main]]
...
[[Languages.ja.menu.main]]
...
The method of setting menu contents will be presented in the Create a section page.
