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).

JS Morisset

Provides a convenient locale / language / region selector in the WordPress admin toolbar.
Perfect for anyone that needs to switch languages quickly and easily:
Allows logged-in users to easily change their preferred locale / language / region setting right from the toolbar menu, instead of having to modify their user profile settings.

The default behavior of WordPress is to apply the user locale preference to the admin back-end only – the WPSSO User Locale Selector add-on can optionally extend the locale preference to the front-end webpage as well.
<h3>Do you use the Polylang plugin?</h3>
If the Polylang plugin is detected, the locale selector will automatically use the correct Polylang language URLs.
<h3>WPSSO Core Required</h3>
WPSSO User Locale Selector is an add-on for the <a href="https://wordpress.org/plugins/wpsso/" rel="ugc">WPSSO Core plugin</a>, which creates extensive and complete structured data to present your content at its best for social sites and search results – no matter how URLs are shared, reshared, messaged, posted, embedded, or crawled.

WPSSO User Locale Selector

Quick and easy locale / language / region selector for the WordPress admin toolbar.

Preferred Locale vs WPSSO User Locale Selector