Steph

This plugin assumes you have a site in two or more languages, and that you are using a separate WordPress install for each language. It helps you create links between the same pages in different 
language versions.
You should have a pattern of parallel sites with urls like http://stephanie-booth.com/en/, http://stephanie-booth.com/de/, http://stephanie-booth.com/fr/. Maybe the page 
http://stephanie-booth.com/en/about is equivalent to the page http://stephanie-booth.com/fr/a-propos. The plugin adds a DBX box in the edit/create page admin form, named “Other Language Slugs”. In 
this case, you’d edit the “about” page, and paste the fr slug (“a-propos”) in the appropriate field.
The plugin will then display a small link to the page in the other language at the bottom of your page, or in your menu bar if you’re using a Sandbox-based theme.
Careful! This only works for Pages, not posts!
For intelligent user redirection when they land on the home page, you might want to try using the technique described here: http://climbtothestars.org/archives/2007/12/28/browser-language-detection-and-redirection/
<h3>Future Development</h3>
Here’s what I’d like this plugin to do, someday:
<ul>
<li>automatically capture URLs like http://stephanie-booth.com/en/a-propos and redirect them to http://stephanie-booth.com/en/about (that’s what happens when people edit URLs directly, just replacing 
the language code — I do it all the time)</li>
</ul>

Bunny’s Language Linker

Helps you create links between equivalent (but different language) pages between WordPress installations.

yushao

Translatio is an open source tool for internationalization and localization of WordPress based websites. Translatio provides two translation workflows:
<ul>
<li>Live Translation workflow</li>
<li>Full Professional Translation workflow</li>
</ul>
Live translation workflow provides instant translation based on Google Translate engine. Simply configure the extra languages you want to support through the intutuive setup interface and save, then you are done.
Full Professional workflow provides more control and higher translation quality, which involves extracting the text for translation, translating and integrating back into WordPress. More specifically Translatio plugin hosts your translations locally for proof reading, editing and final publication. You can intergrate machine translation of your choice to prepare your contents, then leveraging fully featured translation editor with any 3rd party translation agencies.
Translatio provides multiple ways to handle different development phases of the WordPress websites with intuitive and easy to use interfaces, some features include:
<ul>
<li>Live Translation powered by Google Translate</li>
<li>Support Taxonomies translation, including Categories, Tags</li>
<li>Support Woo Commerce </li>
<li>Full cycle to keep translation locally for proof read, edit and publish.</li>
<li>Support Search Engeine Optimizaion/SEO URL</li>
<li>Support new block Gutenberg editor and classic editor.</li>
<li>Support Google Translate integration with editing capability</li>
<li>Language switcher based on
<ul>
<li>Draggable floating meanu</li>
<li>Sidebar widget</li>
<li>Along with title or description</li>
<li>With any page or post</li>
</ul>
</li>
<li>Language switcher is in language name or flags</li>
<li>Detect browser language setting</li>
<li>Support browser cookie</li>
<li>Premium service available</li>
<li>Live support community</li>
</ul>

Translatio

Make your website multilingual ready at ease with live translation or with support of full translation cycle, with machine translation integration.

Bunny’s Language Linker vs Translatio