error messages

Hmm, I can’t reproduce it. This is how it looks like to me:
http://cl.ly/image/111K1X033x3j

Some of the strings are missing, if you want to help with translation please let me know.

Oh I see. There’s a couple of ways around, first would be to filter plugin locale value like this:

add_filter( 'plugin_locale', 'my_override_fu_locale', 10, 2 );
function my_override_fu_locale( $locale, $domain ) {
	// Return global WPLANG locale for any other plugin
	if ( 'frontend-uploader' !== $domain )
		return $locale;

	// Return English locale for Frontend Uploader
	return 'en_US';
}

Put that code in functions.php of your theme. This is untested and might conflict with other plugins (let me know if that’s the case).

Other option would be deleting de_DE files from languages folder in the plugin folder. Obviously this option is less than optimal because you’ll have to delete them each time the plugin is updated.