your code looks ok to me, so can't easily say why it could 'cause such an thing to happen...
maybe someone else can.
anyways, I took the freedom to edit your code a little to help me understand it more clearly.
code:

 if "00000409" = readvalue("HKEY_CURRENT_USER\Control Panel\International", "Locale")
 $null=writevalue("HKEY_CURRENT_USER\Control Panel\International", "Locale", "00000809", "REG_SZ")
else
 ? "Locale is United Kingdom"
 exit
endif

cheers,

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