I think that window belongs to MSN Messenger and not to Messenger Plus!, right? In that case, we can't do anything about it. We don't translate messenger itself, but Messenger Plus
Maybe you should report it in some forum/news/whatever of Microsoft. I don't know it such site/thing does ever exist
Anyway, I'd suggest you to wait for the final version of Windows Live Messenger 8.0 and see if it's corrected there