The long-wavelength behavior of vibrational modes plays a central role in carrier transport, phonon-assisted optical properties, superconductivity, and thermomechanical and thermoelectric properties of materials. Here, we present general invariance and equilibrium conditions of the lattice potential; these allow to recover the quadratic dispersions of flexural phonons in low-dimensional materials, in agreement with the phenomenological model for long-wavelength bending modes. We prove that for any low-dimensional material, the bending modes can have a purely out-of-plane polarization in the vacuum direction and a quadratic dispersion in the long-wavelength limit. In addition, we propose an effective approach to treat the invariance conditions in crystals with non-vanishing Born effective charges where the long-range dipole-dipole interactions induce a contribution to the stress tensor. Our approach has been successfully applied to the phonon dispersions of 158 two-dimensional materials, opening new avenues for the studies of phonon-mediated properties of low-dimensional materials. The dataset uploaded here contains an AiiDA database for new phonon dispersions of all 245 two-dimensional materials produced in this work and essential data for reproducing the main results of this work. These data include the modified q2r and matdyn code of Quantum ESPRESSO distribution, pseudopotentials used in this work, optimized crystal structures, interatomic force constants and phonon dispersions.