Does exactly what it has to do: change admin language to English, independently from the website's language.
The website's language is set by WPLANG in "wp-config.php". That setting sets the language that is declared in the HTML's "head" section, as well as causes WordPress to use localised strings. USE IT!
However, if you want the Admin section to be in English (trust me, having an RTL Admin, like in Arabic or Hebrew, is a pain...), then this does it, and is extremely light. There are other plugins for this, notably two with 30k+ downloads as of today, but they are very heavy and bloated.