Checkboxes with array model in AngularJS

Here are my checkboxes :

<input type="checkbox" ng-model="data.lang[0]" id="FR" ng-true-value="'FR'" ng-false-value="''"  checked/>FR
<input type="checkbox" ng-model="data.lang[1]" id="NL" ng-true-value="'NL'" ng-false-value="''" />NL
<input type="checkbox" ng-model="data.lang[2]" id="EN" ng-true-value="'EN'" ng-false-value="''" />EN

I'd like to check a language and update my model 'data.lang'

But i have a problem :

If second checkbox is checked i have :
   [null,"NL"]
If the third is checked i have :
   [null,null,'EN']

What i need to have :

If second checkbox is checked :
    ['NL']
If second and third are checked :
    ['NL','EN']
If all are checked :
    ['FR','NL','EN']

I don't know how to update my model like i want could you help me? Thanks