By default, when you navigate to view an English article, the interface components are translated into English.
By using the language selection form #MENU_LANG, this will change the interface elements by default and for the articles in the selected language.
Unless we are already in an article for a specific language, English for example, and therefore already using the English interface and with the language menu displaying "English", then if we request to display that page in French by using the language menu, the URL for the page will now add a parameter lang=fr, but in fact nothing actually changes for the site visitor, with both the article itself and its interface remaining in English: what is actually happening, is that it is the article’s own context which has taken priority over the visitor’s request.
We could also see the opposite case though, displaying the interface in French, but reading the article in English anyway. In order to make the interface behave independently to the current article’s or current section’s language, you will need to define the global variable forcer_lang in the mes_options file:
// enforce the language of the visitor
$GLOBALS['forcer_lang']=true;