The substitutionally disordered nature of gallogermanates creates a distribution of crystal-field strengths and symmetries for dopant ions. This causes the Huang - Rhys factor of the transition to vary from site to site, leading to a strongly broadened luminescence band. This luminescence is blue-shifted and strongly quenched with increasing temperature. Calculations show that this behaviour is due to large variations in the internal conversion rates for ions in different parts of the crystal-field distribution. It is shown that a confinement potential rather than a harmonic potential is required to account for the data.