If the BWS tried to rewrite it into your registry, you should not find that key in your ini but have an entry in your BiG World Setup\Logs - directory.
Either directly in it or more likely in folder named Bak-%sometimestamp%. The files name would be BiG World Install Debug.txt and you should look at the end of it.
You changed your codepage to install some mods. If you do not intend to change your installation, you can use your old codepage No. %somecodepage%.Write that value to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Nls\CodePage, change the key ACP from 1252 to the value from above.
Do you want to set your codepage now?
Edited by dabus, 20 July 2015 - 07:58 AM.