The answer to this question is surprisingly subtle. If multiple forces that add up to zero act on an object, then it does not accelerate. (Although the object experiences angular acceleration if it's spatially extended and the forces produce a nonzero net torque.)
But the reason that its velocity remain constant is not because of Newton's first law - it's because of Newton's second law, which describes how an object behaves under the influence of external forces. While you would get the right answer by naively applying Newton's first law to this situation, it's logically incorrect to do so. That's because the first law is not simply a special case of the second law, as it's often presented to be. Instead, it acts as a definition of inertial frames.
The second law is not a generalization of the first law - when stated precisely, it doesn't make any sense without the first law. That's because talking about forces netting out to zero - or even corresponding to vectors at all - actually implicitly assumes a whole lot of nontrivial empirical results that are contained within the second law when it's stated in full. I discuss those subtleties here.