When new message arrives, then vBET determines what is its language using chosen strategy (configurable by "Google Language Detection"). If it is considered as written in other language that forum default, then message is automatically marked by lang bbcode. In other words - vBET marks the message adding info about its language.
Now when page is displayed each message is shown in language of whole page. So if I'm watching English translation of Italian forum then I see everything in English - even is some of messages are written in Italian, some in English, some in Arabic, and so on. Looking on page you see everything in chosen translation no matter in what languages are original messages for this page.
The only way when it is not working like this its when some message language is not appropriately marked (cause of this differs according to chosen strategy of language determining). In this case, such message have to be edited and lang bbcode have to be added/corrected manually.