I'm not sure about this - it could make sense for plugins, but for core it means we don't pick up updates to existing languages that have fallen below the threshold.

e.g. French was once 100%, it dropped down by 20% when we were developing 1.4 (as strings change, get added) but it's currently at 97%.  I'd still want it, even if it was 90%, since they've improved it from where it was.