<li ng-repeat="lang in langs">
    <label> <input
             type="radio"
             name="language"
             ng-disabled="isDisabled"
             ng-model="langs.name"
             ng-value="$index"
             ng-change="getIndex($index)"
             />
     <span> {{lang.name}} </span>
     </label>
</li>

$scope.langs[0].ngDisabled = true;