You may have an issue, if you change the system defaults to another LANG then anything that runs as local system will adopt the new settting. Any new accounts will also adopt the setting.
Is you webapp in its own application pool with its own users if it is, it will use the LANG settings from that users.
If its running as the default application pool it will probs be running as the account Network Service.
So, you may need to the settings for this account.
The SID for this account is S-1-5-20, so look in :
YOUR settings should be at
I have just tried this on my test server and it worked, so, your step by step guide is :
Use Control Panel -> Regional Settings
Adjust setting if needed.
Under the Advanced, select Apply all settings to the current user account and the default user profile, click ok.
Run regedit edit
Export [HKEY_CURRENT_USER\Control Panel\International] to a file
Open this file in notepad, change :
Import this file, run IISRESET, things should work.
It worked for me, let me know if it works for you.