I had some "wierd" battery behaviour with my L920 on Monday after installing the update. As with you, after a reboot to change language, it dropped from 60ish to 30ish - even on charge, the battery percentage was going down. Then, though, I rebooted and it jumped from 17% to 70%!
Yesterday, though, things were much more "normal". I tend to have my phone on a desktop charging stand most of the day at work, but when I got home last night the drain throughout the evening was similar to what I'd expect given the amount I was playing with the phone!
I seem to recall having something similar after installing the Dev preview of GDR3 (in other words, it took a day or so for the battery meter to re-calibrate).