What would be the easiest, cheapest approach for this?
Simple, create one form per language.
In functions.php
or, preferably, as a custom plugin:
add_shortcode( 'my-lingo-form', 'shortcode_wpse_98360');
function shortcode_wpse_98360()
{
$lingo = your_language_detection_method();
switch( $lingo )
{
case 'en':
echo do_shortcode('[form-en]');
break;
default:
echo do_shortcode('[form-other-languages]');
break;
}
}
In your post or page: [my-lingo-form]
.