I don't know how cleanly memory adopts a default state upon battery failure but even at worst you can still use the machine almost as normal: hold down R while switching on to reset the CMOS RAM. You'll be told that it is reset and prompted to press break for a warm reboot, at which point you should be back to factory defaults and safe at least until your next power cycle.
*ST.) can be used to see what settings are currently stored;
*CO.) can be used to set CMOS values. If status reports a line like:
i.e. use the ROM in slot 0 as the default language, then you can set that by:
*CONFIGURE LANG 12
i.e. use the ROM in slot 12 as the default language. The built-in BASIC ROM appears in slot 12.
NOTE: A BBC Master 128 with the default Megabit ROM chip will, upon resetting the CMOS and doing a warm reset, drop the machine into ADFS. If there are no disk devices available, then the BBC will hang until the user aborts by pressing