Sign Up form mixes languages if forum localization was changed in another tab

Preconditions
Open http://qa.jtalks.org/jcommune/ page in two tabs

Steps to reproduce

1. Choose Russian localization in tab 1
2. Choose English localization in tab 2
3. Click "Sign Up" link and submit the form by pressing Enter key in tab 1 (with Russian localization)

Actual result
Sign Up form mixes languages if forum localization was changed in another tab:
error messages and Captcha field placeholder are in English.
Expected result
Sign Up form doesn't mix languages if forum localization was changed in another tab:
all error messages and placeholders are in Russian

*The wrong behaviour reproduces in all localization combinations

http://screencast.com/t/lI4unsyU
http://screencast.com/t/91YRFy0zQ