1. These should be set in html head of your blog (Design -> Edit HTML, insert after <head>)
<script src='http://ajax.googleapis.com/ajax/libs/jquery/1.5.2/jquery.min.js' type='text/javascript'/>
<script src='http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.12/jquery-ui.min.js' type='text/javascript'/>
<link href='http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.12/themes/base/jquery-ui.css' rel='stylesheet' type='text/css'/>
<script src='http://csjs.googlecode.com/files/csjs_1.1.2.js' type='text/javascript'/>
<script src='http://csjs.googlecode.com/files/csjs.browser_1.0.3.js' type='text/javascript'/>
<script src='http://csjs.googlecode.com/files/csjs.i18n_1.2.3.js' type='text/javascript'/>
<script src='http://csjs.googlecode.com/files/csjs.jquery_1.0.0.js' type='text/javascript'/>
<script src='http://csjs.googlecode.com/files/csjs.ui.control_1.0.4.js' type='text/javascript'/>
<script src='http://csjs.googlecode.com/files/csjs.ui.control.i18n_1.0.2.js' type='text/javascript'/>
<script src='http://csjs.googlecode.com/files/csjs.ui.control.i18n.admin_1.0.0.js' type='text/javascript'/>
2. Add language selector as a gadget (Design -> Page Elements -> Add a Gadget -> Basics -> HTML/JavaScript).
<div csjs-ctrl-class="csjs.ui.control.i18n.ListLanguageSwitch"></div>
3. Edit your post HTML (Posting -> Edit Posts -> Edit -> Edit HTML)
Copy original post HTML to 'Original HTML' tab of the tool.
Go to 'I18N Injected HTML' tab, add needed translations. Note: language keys is predefined values ISO Codes.
Replace original post HTML on blogger with new I18N injected.
NOTE: Edited HTML is not supported with blogger core post composer! Thus ones injected i18n you will be unable to continue to work with Blogger composer on edited post.
Current I18N implementation as javascript plugin is workaround of missing i18n server-side support on Blogger.