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I don't know what you're getting from Mathematica. There may be an issue with getting Sage to use the initial conditions on the derivative. I've removed the initial conditions in your command and used substitutions to take care of the initial conditions.

The following code all indicates that the solutions are right.

ans=desolve_system([X,Y],[x,y],ivar=t)
x1(t)=ans[0].rhs().subs(x(0)==0).subs(diff(x,t,1)(0)==v0*cos(T))
print x1(t)
print x1(0)
print diff(x1,t)(0)
diff(x1(t),t,2)+G*diff(x1(t),t,1)

and

y1(t)=ans[1].rhs().subs(y(0)==0).subs(diff(y,t,1)(0)==v0*sin(T))
print y1(t)
print y1(0)
print diff(y1,t)(0).simplify_full()
lhs=diff(y1(t),t,2)+G*diff(y1(t),t,1)+g
lhs.simplify_full()