Always trust user This option is based on the settings of the user who is typing a message. So google can recognize the message language by user settings. If user have his language not set - google can't translate it.
If user will make a message in language other that in his admincp - google still can't translate it.
This option is for Optimizations and if this option is enabled - language tags will not be added.

Please go to your admincp->vBET->google and
1. set Google Language Detection to Trust Google when result is trust worthy
2. set Google Language Detection Confidence to 0.1
3. test it! (make a new message)
Should works good.
IMPORTANT - you need to pack old posts in lang tags by hand.