James

WPTranslator plugin allows you to add a widget to your site providing translations in to multiple languages.
If someone visits your site and wants to translate it they can simply click a flag or select a language from the dropdown box to translate the text into their preferred language.
To view the options available activate the plugin and then go to Settings > WPTranslator
We have kept it really simple with just some styling options.
Visit the main website for news and additional information: <a href="http://wptranslator.com/" title="Wordpress Translator" rel="nofollow ugc">WordPress Translator</a>
It is simple to install and you can place a widget anywhere on your site with flags or a drop down box which when clicked translates your site into the language specified.
If you like the plugin please leave a review. Any suggestions or ideas for how we can improve the plugin would be greatly appreciated.
Full list of languages available for translation at <a href="http://wptranslator.com/full-list-of-translation-languages-available/" title="full list of translation languages" rel="nofollow ugc">WordPress Translation Languages</a>

WPTranslator

WPTranslator allows visitors to translate your site into any language

unclego

Lets you have your backend administration panel in english or any installed language, even if the rest of your blog is translated into another language. Language preferences can be set per user basis in user profile screen.
Tested on MultiSite.
Deals with frontend ajax requests
Notice: 
After release of WordPress 4.7 this plugin is a bit obsolete.
In WordPress 4.7 users are able to select their preferred locale (language) when editing their profile. Plugin add exact same functionality to pre 4.7 WordPress.
I still maintain it for people with outdated WordPress sites. And the plugin is still usefull for sites with outdated plugins that not obey get_user_locale(). 
<a href="https://make.wordpress.org/core/2016/11/07/user-admin-languages-and-locale-switching-in-4-7/" rel="nofollow ugc">More info</a>

Admin Language Per User

a must have for multilingual sites

Switch your backend administration panel in any installed language per user basis.

WPTranslator vs Admin Language Per User