I don't know of a command line utility that strictly does this, but you could use a small bit of VBScript to change the associated registry values. It's not as straightforward as changing the locale in the General tab of the Regional and Language Options dialog, but it works.
For example, to change the settings to match "English (United Kingdom)" you can use something like this:
Dim WSHShell
Set WSHShell = CreateObject("Wscript.Shell")
WSHShell.RegWrite "HKCU\Control Panel\International\iCountry", "44", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\iCurrDigits", "2", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\iCurrency", "0", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\iDate", "1", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\iDigits", "2", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\iLZero", "1", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\iMeasure", "0", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\iNegCurr", "1", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\iTime", "1", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\iTLZero", "1", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\Locale", "00000809", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\s1159", "AM", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\s2359", "PM", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sCountry", "United Kingdom", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sCurrency", "£", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sDate", "/", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sDecimal", ".", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sLanguage", "ENG", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sList", ",", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sLongDate", "dd MMMM yyyy", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sShortDate", "dd/MM/yyyy", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sThousand", ",", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sTime", ":", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\DefaultBlindDialFlag", "00", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sTimeFormat", "HH:mm:ss", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\iTimePrefix", "0", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sMonDecimalSep", ".", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sMonThousandSep", ",", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\iNegNumber", "1", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sNativeDigits", "0123456789", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\NumShape", "1", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\iCalendarType", "1", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\iFirstDayOfWeek", "0", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\iFirstWeekOfYear", "0", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sGrouping", "3;0", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sMonGrouping", "3;0", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sPositiveSign", "", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\sNegativeSign", "-", "REG_SZ"
WSHShell.RegWrite "HKCU\Control Panel\International\Geo\Nation", "244", "REG_SZ"