Hook up localized messages.json to website

Right now we use the navigator language to determine localization and
replace the website contents with translated strings.

proxy/static/index.js

@@ -0,0 +1,60 @@
+class Messages {
+  constructor(json) {
+    this.json = json;
+  }
+  getMessage(m, ...rest) {
+    if (this.json.hasOwnProperty(m)) {
+    let message = this.json[m].message;
+    return message.replace(/\$(\d+)/g, (...args) => {
+      return rest[Number(args[1]) - 1];
+    });
+    }
+  }
+}
+
+
+defaultLang = "en_US";
+
+var getLang = function() {
+  let lang = navigator.language || defaultLang;
+  lang = lang.replace(/-/g, '_');
+  if (availableLangs.has(lang)) {
+    return lang;
+  }
+  lang = lang.split('_')[0];
+  if (availableLangs.has(lang)) {
+    return lang;
+  }
+  return defaultLang;
+}
+
+var fill = function(n, func) {
+  switch(n.nodeType) {
+    case 1:  // Node.ELEMENT_NODE
+      const m = /^__MSG_([^_]*)__$/.exec(n.dataset.msgid);
+      if (m) {
+          val = func(m[1]);
+          if (val != undefined) {
+            n.innerHTML = val
+          }
+      }
+      n.childNodes.forEach(c => fill(c, func));
+      break;
+  }
+}
+
+console.log("Fetching", `./_locales/${getLang()}/messages.json`);
+
+fetch(`./_locales/${getLang()}/messages.json`)
+  .then((res) => {
+    if (!res.ok) { return; }
+    return res.json();
+  })
+  .then((json) => {
+    messages = new Messages(json);
+        console.log("Filling document body");
+    fill(document.body, (m) => {
+        console.log("Filling ", m);
+      return messages.getMessage(m);
+    });
+  });