SPT0346-52 is one of the most most luminous and intensely star-forming galaxies in the universe, with LFIR > 10¹³ L_⊙ 13 and Σ _SFR ≈ 4200 M_⊙ yr^-1 kpc ^-2. In this paper, we present ∼0.′15 ALMA observations of the [C II] 158 μm emission line in this z = 5.7 dusty star-forming galaxy. We use a pixellated lensing reconstruction code to spatially and kinematically resolve the source-plane [C II] and rest-frame 158 ?m dust continuum structure at ∼700 pc (∼0.′ 12) resolution. We discuss the [C II] deficit with a pixellated study of the L[C II]/LFIR ratio in the source plane. We find that individual pixels within the galaxy follow the same trend found using unresolved observations of other galaxies, indicating that the deficit arises on scales ≲700 pc. The lensing reconstruction reveals two spatially and kinematically separated components (∼1 kpc and ∼500 km s^-1 apart) connected by a bridge of gas. Both components are found to be globally unstable, with Toomre Q instability parameters ≪1 everywhere. We argue that SPT0346-52 is undergoing a major merger, which is likely driving the intense and compact star formation.