We have investigated the photoluminescence (PL) of two carefully selected dilute nitride Ga1−xInxNyAs1−y multiple quantum well structures in magnetic fields up to 50 T as a function of temperature and excitation power. The observation of a nonmonotonic dependence of the PL energy on temperature indicates that localized states dominate the luminescence at low temperature, while magneto-PL experiments give new insights into the nature of the localization. We find that the low-temperature spatial distribution of carriers in the quantum well is different for electrons and holes because they are captured by different disorder-induced complexes that are spatially separated. A study of the thermalization of the carriers toward free states leads to the determination of the free-exciton wave-function extent in these systems and enables an assessment of the localization potentials induced by inhomogeneity in the quantum well.