config.toml を以下のように修正した。
+++ b/hugo/config.toml
@@ -2,7 +2,7 @@ baseURL = "http://www.example.com"
theme = "hugo-coder"
-languageCode = "en"
+languageCode = "ja"
-defaultContentLanguage = "en"
+defaultContentLanguage = "ja"
paginate = 20
...
-[languages.en]
+
+[languages.ja]
languageName = "🇬🇧"
-[[languages.en.menu.main]]
+[[languages.ja.menu.main]]
name = "About"
weight = 1
url = "about/"
-[[languages.en.menu.main]]
+[[languages.ja.menu.main]]
name = "Blog"
weight = 2
url = "posts/"
-[[languages.en.menu.main]]
+[[languages.ja.menu.main]]
name = "Projects"
weight = 3
url = "projects/"
-[[languages.en.menu.main]]
+[[languages.ja.menu.main]]
name = "Contact me"
weight = 5
url = "contact/"
ドキュメント的には、次のページが参考になる。