How about FORTRAN--say FORTRAN II or USA BASIC FORTRAN? Very small
language; very few statement types and historically correct. Can be
(and was) implemented on just about anything. You don't need
registers, stacks or any of that fancy stuff.
Seems that a dialect (LRLTRAN) was used to bootstrap operating
systems at Lawrence Livermore back in the day.
Some tiny BASICs also come to mind.
Cheers,
Chuck