  At zero temperature, a Galilean-invariant Bose fluid is expected to be fully
superfluid. Here we investigate theoretically and experimentally the quenching
of the superfluid density of a dilute Bose-Einstein condensate due to the
breaking of translational (and thus Galilean) invariance by an external 1D
periodic potential. Both Leggett's bound fixed by the knowledge of the total
density and the anisotropy of the sound velocity provide a consistent
determination of the superfluid fraction. The use of a large-period lattice
emphasizes the important role of two-body interactions on superfluidity.
