Your code updates the localization, but the new messages will work only on the next update.
Replace switchLanguage with this:
this.switchLanguage = function() {
// If I localize later, nothing is changed.
ko.validation.localize({required: 'eh... sorta?'});
for (var prop in this)
if (ko.isObservable(this[prop]) && typeof(this[prop].valueHasMutated) === 'function')
this[prop].valueHasMutated();
};