Sam Bull

Sets the site locale dynamically according to visitor preference. 
This preference comes from the user’s browser.
To install all available translations click the ‘Install All’ button in Settings->General. 
To install individual locales, select the site language and save changes for each locale you want to install.
Technical details: 
 Will parse the Accept-Language HTTP header and find the best matching language for the user from the translations available on the site.

Preferred Locale

Select site locale/language according to user preference (set in their browser).

jomo

Adds Translate button to the WordPress admin bar to translate any selected text. 
Adds Translate buttons to BuddyPress and bbPress messages.
Translate translates selected text to current locale: where user is logged in, this is the user’s preferred language.
Perfect for a multilingual Buddypress site, this tool allows your users to translate messages to their own language.
Other tools translate the whole site but a polyglots and language learners may not want this, for learning it is best to see the original, and translate only where necessary. 
Or the whole site may be already translated to your preferred language but you are getting messages from other users (or from some untranslated plugin) and you want to translate those.
Currently uses Google Translate.
<h3>Contribution</h3>
If you have a feature request, or if you have developed the feature already, please feel free to use the <a href="https://github.com/Jon007/jm-buddy-translate/issues" rel="nofollow ugc">Issues</a> and/or (Pull Requests)[https://github.com/Jon007/jm-buddy-translate/pulls] section.
Plugin translations are provided for English, French, Spanish, Chinese, please feel to contribute more <a href="https://translate.wordpress.org/projects/wp-plugins/jm-buddy-translate" rel="nofollow ugc">translations</a>.

JM Buddy Translate

Quick and easy translate BuddyPress and bbPress messages or any selected text.

Preferred Locale vs JM Buddy Translate