I found this while cruising my tech news on dailyrotation. It is from WinXPcentral.com.
It works. I did it just a few minutes ago.
Click Options in the upper right, click General Preferences, click Account Information in the box on the left, retype your password if asked, click Edit for Member Information, then under General Preferences click the link for Language & Content. Use the drop down menu to choose Germany, France, or UK. Click Finished.
Enjoy!
The Tod sent this my way and after trying it himself he got in.
To get Yahoo Mail Beta, just switch your content preference to Germany, France or UK. Then you will be asked if you want to join the beta when you log into your Yahoo Mail. Say yes, and join the beta. Then from the options menu, change your content preference what it was before. Then go to Yahoo Mail again. You should see Yahoo Beta. If you don't see it, go to options and click "Try Beta" button. That's all.
It works. I did it just a few minutes ago.
Click Options in the upper right, click General Preferences, click Account Information in the box on the left, retype your password if asked, click Edit for Member Information, then under General Preferences click the link for Language & Content. Use the drop down menu to choose Germany, France, or UK. Click Finished.
Enjoy!