Add support for translatable placeholder strings in form content

[dannyvankooten]

Eg using WPML...

// upon saving form, parse list of placeholders & register each with WPML String Translator
icl_register_string( $context, $name, $default_text );

// upon showing form, get translated string for each placeholder
icl_translate( $context, $name, $default_text );

For now, people can duplicate the form and create a separate form for each site language.