ab-tools

A simple plug-in that displays flag icons.
This plug-in does not provide a way to translate posts directly, but offen you do not want a direct translation of posts. Still you want to add flag icons that link to other language versions of your blog.
After installation of this plug-in a new widget will be added that you can use to display such flag icons on your blog. A set of flag icons is included, but if you want to use your own images that’s possible, too.
<h4>Additional info, support and developer contact:</h4>
<ul>
<li>English: http://www.ab-weblog.com/en/wordpress-plug-ins/flags-widget/</li>
<li>German: http://www.ab-weblog.com/de/wordpress-plug-ins/flaggen-widget/</li>
</ul>
Please let me know if you find a problem with this plug-in or have questions!
<h4>You can see this plug-in in action on posts of this blog:</h4>
<ul>
<li><a href="http://www.ab-newsonline.com/en/" title="Read Current News Online at AB-NewsOnline.com" rel="nofollow ugc">Read Current News Online at AB-NewsOnline.com</a></li>
</ul>
If you like this plug-in, please <a href="https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=donation%40breitschopp.de&item_name=Donation%20for%20Flags%20Widget%20WordPress%20Plug-In&no_shipping=0&no_note=1&tax=0&currency_code=EUR&lc=CA&bn=PP%2dDonationsBF&charset=UTF%2d8" title="Donate With PayPal" rel="nofollow ugc">consider a donation</a> to keep it free. Thank you! 
… and please don’t forget the 5 star rating, if you like this plug-in. 🙂

Flags Widget

A simple plug-in that displays flag icons that link to other language versions of your blog.

julienvdg

hreflang Flag plugin uses <a href="http://www.w3.org/TR/css3-selectors/#attribute-selectors" title="W3C CSS3 Attribute selectors description" rel="nofollow ugc">css3 selectors</a> to add a flag icon to your links depending on their <a href="http://www.w3.org/TR/html401/struct/links.html#adef-hreflang" title="hreflang attribute description on W3C" rel="nofollow ugc">hreflang attribute</a>.
This is really useful if you regularly link to pages in other languages and you want to inform your visitors about the language of the link target.
It’s even more useful if your own blog is multilingual.

hreflang Flag

Add a flag icon to link corresponding to the hreflang attribute.

Flags Widget vs hreflang Flag