First pass at expanded i18n doc....

chrome/common/extensions/docs/static/manifest.html

 <div id="pageData-name" class="pageData">Formats: Manifest Files</div>
 <div id="pageData-showTOC" class="pageData">true</div>
 
 <p>
 Every extension has a
 <a href="http://www.json.org">JSON</a>-formatted manifest file,
 named <code>manifest.json</code>,
 that provides important information about the extension.
 </p>
 
 <h2 id="overview"> Field summary </h2>
 
 <p>
 The following code shows the supported manifest fields,
 with links to the page that discusses each field.
 The only fields that are required for every extension
 are <b>name</b> and <b>version</b>.
 </p>
 
 <pre>
 {
   <em>// Required</em>
   "<a href="#name">name</a>": "<em>My Extension</em>",
   "<a href="#version">version</a>": "<em>versionString</em>",
 
   <em>// Recommended</em>
   "<a href="#description">description</a>": "<em>A plain text description</em>",
   "<a href="#icons">icons</a>": { ... },
+  "<a href="#default_locale">default_locale</a>": "en",
 
   <em>// Pick one (or none)</em>
   "<a href="browserAction.html">browser_action</a>": {...},
   "<a href="pageAction.html">page_action</a>": {...},
   "<a href="themes.html">theme</a>": {...},
 
   <em>// Add any of these that you need</em>
   "<a href="background_pages.html">background_page</a>": "<em>aFile</em>.html",
   "<a href="override.html">chrome_url_overrides</a>": {...},
   "<a href="content_scripts.html">content_scripts</a>": [...],
@@ skipping 56 matching lines @@
 If you submit your extension to the
 <a href="https://chrome.google.com/extensions">gallery</a>,
 you'll need to upload additional images,
 including a 32x32-pixel logo
 and at least one screenshot of your extension.
 For more information on gallery requirements,
 see the
 <a href="http://www.google.com/support/chrome/bin/answer.py?answer=113909">gallery help</a>.
 </p>
 
+<h3 id="default_locale">default_locale</h3>
+
+<p>
+Specifies the subdirectory of <code>_locales</code>
+that contains the default strings for this extension.
+This field is <b>required</b> in extensions
+that have a <code>_locales</code> directory;
+it <b>must be absent</b> in extensions
+that have no <code>_locales</code> directory.
+For details, see
+<a href="i18n.html">Internationalization</a>.
+</p>
+
 <h3 id="name">name</h3>
 
 <p>
 A short, plain text string
 (no more than 45 characters)
 that identifies the extension.
 The name is used in the install dialog,
 extension management UI,
 and the <a href="https://chrome.google.com/extensions">extension gallery</a>.
 </p>
@@ skipping 91 matching lines @@
 A missing integer is equal to zero.
 For example, 1.1.9.9999 is newer than 1.1.
 </p>
 
 <p>
 For more information, see
 <a href="autoupdate.html">Autoupdating</a>.
 </p>
 
 <!-- [PENDING: Possibly: point to the gallery and make a big deal of the fact that autoupdating is free if you use the gallery.] -->