I can think of some languages that let you use variables uninitialized

Java initializes numerical types to zero.

In C,

int i;

makes i take up the sizeof(int) bytes on the stack but it is filled with whatever garbage was there earlier.

However, both of these languages require you to declare the type of a variable.

Some FORTRAN 77 compilers might let you use uninitialized variables as zero. Of course, new Fortran programs should probably use implicit none, and most compilers for both C and Fortran let you force warnings for uninitialized variables.