forcejapanese.js

// ==UserScript==
// @name          enforce japanese
// @include       *
// ==/UserScript==

(function () {
  var html = document.getElementsByTagName("html")[0];
  if (!html) return;
  if (!html.lang) {
    var c = document.characterSet;
    if (!c) {
      html.lang = "ja";
    } else {
      switch (c) {
      case "utf-8":
      case "utf8":
      case "UTF-8":
      case "UTF8":
      case "utf_8":
      case "UTF_8":
        html.lang = "ja";
      }
    }
  }
})();