Language Picker plugin and Login problems

Hi,

Yesterday, I installed Language Picker plugin downloaded from this location
https://noprerequisite.com/2004/06/07/language-picker-plugin-for-wordpress/
It worked well.
Then after a few hours of working on my WordPress blog I did logout and went sleep.

Today morning I tried to login my blog and I noticed after I click to Login button I got blank page under URL https://mateusz.loskot.net/wp-admin/

First I started to walk through WordPress forums searching for login problems with WP 1.5.x. I tested many solutions, deleting cookies and cache, deleting all content from my database, etc. but I didn’t associate the problem with the Language Picket plugin I installed yesteday.
After some time I recalled that plugin. So, I removed the plugin from my WP installation and now everything works very well.

That’s my little story for other users.

I also have a question how should I go to try to repair Language Picker plugin? Any debuging tips? What should I check at first?
I’m 3-days user of WP so I don’t know all of WP architecture details.

Cheers