WordFez is a plugin for bilingual (French-English) posts. It is seamlessly integrated into WordPress using a TinyMCE plugin.
It will add 2 extra buttons in the WYSIWYG interface for French and English language. Each button inserts 2 anchors to surround each language. The anchors are labeled by small flags. 
The usage is very straight-forward. The plugin comes with a widget to allow each reader to conveniently and instantly switch between the languages.
You need WordPress 2.5 at least.
New in v0.2.1:
<ul>
<li>Compatibility with WordPress 2.5</li>
<li>Fix a small problem in ‘alt’ fields</li>
</ul>

WordFez

WordFez is a plugin for bilingual (French-English) posts. It is seamlessly integrated into Wordpress using a TinyMCE plugin.

Fahad Alduraibi

Do you write posts in different languages that have different directions? 
This plugin enables a feature in the post editor which allow writing texts in Left-to-Right and Right-to-Left directions in the same post (or page) so you can write in English (and any other LTR language) and also RTL languages like Arabic, Persian and Hebrew.
Simply look for the new buttons added to the post editor after installing the plugin. (See the screenshots for better understanding)
اضافة زرارين لشريط التحرير لتمكين تحويل النص للقراءة من اليمين لليسار مثل اللغة العربية والفارسية والعبرية
<a href="http://www.fadvisor.net/blog/2008/10/wp-rtl/" rel="nofollow ugc">More details</a>
<ul>
<li>Note: This plugin will not change the direction of the post title, since that is controlled by the theme.</li>
</ul>

WP-RTL

Adds two buttons to the TinyMCE editor to enable writing text in Left to Right (LTR) and Right to Left (RTL) directions.

WordFez vs WP-RTL