Q:

Setting the document title with Meteor

I'm just starting with Meteor.  In an app which is to be localized, I want to set the document title. 
I am following the advice given by Bernát
In my barebones version, I have just 2 documents:
head.html
<head>
  <meta charset="utf-8">
  <title>{{localizedTitle}}</title>
</head>

ui.js
UI.registerHelper("localizedTitle", function() {
  var title = "Localized Title"
  document.title = title;
});

When the app loads, the document title is "{{localizedTitle}}".  If I call UI._globalHelpers.localizedTitle() from the console, the correct title is shown. 
What do I have to do to get the localized title to show when the page is loaded? 

EDIT: This works for me, but it seems to be a bit of a hack.  The title template does nothing but get itself rendered, which actually adds nothing to the interface. 
body.html
<body>
{{> title}}
</body>

<template name="title">
</template>

title.js
Template.title.onRendered(function () {
  document.title = getLocalizedString()

  function getLocalizedString() {
    return "Title : in English"
  }
})

A:

Following Bernát's answer, your global helper should not be called in the head's <title> tag, but within the <template> tag of the template where you wish to have a given title.  In Meteor, <head> does not count as a template, therefore you cannot use Spacebars notation in it: it will just be considered as simple text. 
Also, keep in mind that your helper will not return (i.e. print) anything to the page.  document.title = "something" directly assigns "something" to your ` tag.  So no need to call your helper inside it! 
So, say you want to have the "Localized Title" title for a page using the localized template :
<template name="localized">
  <h1>This is the localized page</h1>
  {{localizedTitle}}
</template>

Here, your trick should work.