Thanks for your respons.
I figured it out, I made another if statement within like this:
<?php
if( get_field('te_koop') )
{
if( qtrans_getLanguage() == 'en' ){
echo '<span class="sale">For sale</span>';
}
else
{
echo '<span class="sale">Te koop</span>';
}
}
else
{
echo '';
}
?>