bpo-31149: Doc: Add Japanese to the language switcher. (#3028)

diff --git a/Doc/tools/static/switchers.js b/Doc/tools/static/switchers.js
index 470ff6d..de9e7b7 100644
--- a/Doc/tools/static/switchers.js
+++ b/Doc/tools/static/switchers.js
@@ -21,6 +21,7 @@
   var all_languages = {
       'en': 'English',
       'fr': 'Français',
+      'ja': 'Japanese',
   };
 
   function build_version_select(current_version, current_release) {