Well if you would have looked at the ispconfig.org page under download. You would have seen a link to "ISPConfig 3 mailuser interface for roundcube" which links to my plugins for Roundcube.
This plugins provide everything what a user should be able to do and can also be enabled one by one. So if you only want forwarding, than you activate jquerui, ispconfig3_account and ispconfig3_forward.
PS: See docs for install instructions
PPS: It also includes already the hungarien language files, thought I don't know if they are accurate or up to date