Plus or minus polyglots!

Python 2 / 3, 52 bytes

lambda l:sum(l[:1]+[x*int(1-(1/2)*4)for x in l[1:]])

int(1-(1/2)*4) returns 1 in Python 2 because 1/2 evaluates first to 0, and then 0 * 4 = 0.

int(1-(1/2)*4) returns -1 in Python 3 because 1/2 evaluates first to 0.5, and then int(0.5 * 4) = int(2.0) = 2.

C and C++ (both from GCC), 81 75 73 bytes

int c;int f(int*a,int l){auto d=.5;c=*a++;for(;--l;a++)c+=*a-4*d**a;c=c;}

Takes a pointer to array and length.

Explanation: _{still using @Steadybox' explanation :p} In C, auto d=.5 declares an integer variable with the auto storage class (which is the default), which is then initialized to 0, whereas in C++11 it declares a double, which is initialized to 0.5.

C - plus language: Try it online!

C++ - minus language: Try it online!

Jelly / 05AB1E, 3 bytes

00000000: c6 71 53                                         .qS

This is a hexdump (xxd) of the submitted program.

Try it online!

Jelly: Sum

Jelly uses the Jelly code page, so it sees the following characters.

İqS

Try it online!

How it works

İqS  Main link. Argument: A (array)

İ    (ignored)
 q   Unrecognized token. This breaks the link; nothing to the left is executed.
  S  Take the sum of A.

05AB1E: Difference

05AB1E uses Windows-1252, so it sees the following characters.

ÆqS

Try it online!

How it works

Æ     Take the difference of the input.
 q    Quit.
  S   (ignored)