what do you mean you want a button to switch to english?
<a href="/en/" title="Switch to English">Switch to English</a>
will suffice? or even a little British flag etc
For the geo stuff, download Maxmind's database and install as an apache module, then (in php)
if ($_SERVER['GEOIP_COUNTRY_CODE'] != 'BE') {
header("Location: /en/");
exit();
}
or if installing as apache modules boggles the brain to much, use geoplugin...
$geolocation_data = unserialize(file_get_contents('http://www.geoplugin.net/php.gp?ip=xx.xx.xx.xx'));
if ($geolocation_data['geoplugin_countryCode'] != 'BE') {
header("Location: /en/");
exit();
}