This plugin adds a standard compliant language tag to your post and page.
If you can write your posts in 2 or more(Woo...) languages within one blog, you might want to tell the client browser the language attribute of you posts.
According to the tutorial on W3C, language attribute can be declared in the container html tag of your post like this: <div id="post_id" class="post_entry" lang="zh-Hant" xml:lang="zh-Hant" dir="ttb">... Here "zh-Hans" is the language attribute of your post which means it's written in Simplified Chinese.
This plugin gives your post a language tag by adding a meta deta to that post.
To add language tag, select it from the language box when you compose/edit a post. Select "None" will delete the language tag.