OzTheGreat

Adds a multiple select box to every non-admin user profile that allows admins to 
select which languages that user can see / edit in the wp-admin.
Languages that a user cannot see are hidden. If they try to switch surreptitiously 
it will throw an error message.
This plugin REQUIRES WPML and is useless without it.

WPML Editor Languages

Allows admins to restrict users to specific languages in the WordPress admin

Katsushi Kawamori

<h4>Automatically switch the locale according to the locale of the browser.</h4>
<ul>
<li>Can specify multiple locale of browser.</li>
<li>Can import and export browser locale and WordPress locale configuration file with CSV.</li>
</ul>

Locale Auto Switch

Automatically switch the locale according to the locale of the browser.

WPML Editor Languages vs Locale Auto Switch