Translating The site not the dashboard

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I have managed to cobble together a messages.po file which appears to be working when I switch the language in the dashboard to be French but I only want the text to be translated when in the French section of the site?

I also notice If I switch to French parts of the English section are then translated to French.

How do I serve French languages when in a French section?

I have followed all the tutorials on site translations some I got to work some I could not but I can not work out how to keep the site dashboard in English and change only the site sections

My current attempt looks something like this
switch(MultilingualSection::getCurrentSection()->msLanguage){
    case "fr":
        define('LOCALE', 'fr_FR');
//I have also tried 
//Localization::changeLocale('fr_FR');
        break;
    default :
        define('LOCALE', 'en_GB');
        break;
}
?>
<!DOCTYPE html>
<html lang="<?=LOCALE?>">


This sets the locale constant but does not run the Language files

I should point out that I have tried following the how to over here
http://www.concrete5.org/documentation/how-tos/developers/translati...

over here
http://www.concrete5.org/documentation/how-tos/developers/how-to-tr...

and
http://www.concrete5.org/documentation/how-tos/developers/generate-...

I can see the language files within the dashboard but require them to translate depending on the section of the site

TheRealSean