Add japanese to translations (#27599)

Author: okazou

site/_data/translations.yml

@@ -17,3 +17,8 @@
   code: pt-BR
   description: Bootstrap 4 Português do Brasil
   url: https://getbootstrap.com.br/v4/
+
+- name: Japanese
+  code: ja
+  description: Bootstrap 4 日本語リファレンス
+  url: https://getbootstrap.jp/