This plugin allows you to write posts in more than one language, all you have to 
do is to add some divs to your posts, each div with a class, that is the 
language’s name. This way your readers will be able to just click in the flags 
and change between languages.

WPMultiLanguageChanger

This plugin allows you to write and you readers to change between languages in your blog in a easy way.

Fahad Alduraibi

Do you write posts in different languages? 
Would you like your blog theme language to be the same as the language in which your wrote your post or page?
With this plugin you can set the language of your blog per post and also per page, that is if your blog is in English and you wrote a post or a page in Spanish or Arabic you can set the language of your blog to match that post’s or page’s language, so when your visitors open that post or page they will see your blog in that language. You don’t have to translate your theme and plugins if they come with translations and even it will change the direction of the view to RTL (Right-to-Left) if your language is RTL based.
<ul>
<li>The plugin does not provide any translations, it only changes the themes and plugins translation language if they provide there own translations. </li>
<li>The available languages are only the languages that WordPress already recognizes and supports.</li>
<li>The plugin only works for posts and pages, the front page will show in the blog default language.</li>
</ul>

Per Post Language

This plugin does a great job when all you want is to have a page in a different language. Since it changes the WordPress locale, it affects the HTML language attributes, the language attributes, generated by SEO/OpenGraph/sharing plugins, etc. which is great.

I was just researching the proper action hook to use to switch the locale per post or page, when I found out it has been already done and packaged in plugin. It saved me an hour or two developing my own solution.

Does just what it says

This plugin allows the user to set the blog language per post or page while having a default blog language.

WPMultiLanguageChanger vs Per Post Language