I have not yet been able to reproduce this. I'll try on my localhost.
I upgraded a forum from 1.1.4 (clean minus the installation of tinyportal and felblog, which were both removed prior to upgrade) to 2.0 beta 3 public today.
As I was going over the user preferences, I noticed the drop down language selection under Account Settings was set on Persian. I quickly changed that to English, but when I did so, numerous text strings disappeared, likely all of the text strings new to 2.0.
I'm not sure if this is the manner which I uploaded the files (didn't want to bother with a stand-alone ftp program, so I used Cpanel's filemanager (new one in v11), extracted the upgrade archive to a different folder and moved the new files into the current forum.
Also possible that it was due to the extremely sluggish upgrade process. Possible that's due to my host having some problems over the past few day.
I'd love to chalk this one up to "human error/act of the internet" but I don't want to leave it as such if this is being experienced by others.
And yeah, this should go in bug reports, but I'd want to confirm it as such before it's moved.