The thing you're trying to achieve is not what Diem default admin generator is designed to do AFAIK.
You can though write your own view/template for this model form (thus opt out of using yml configured generated views) and a custom controller to handle save action.
It's not the easiest way (It's not that hard either) and it's quite imperative, but I don't know if any other, much easier exists.
If you're done with configuring you can grab generated partial from cache folder, modify it (by adding locale fields). Then you'd have to extend the controller and write your own save logic. It won't take long this way. All you have to remember - this particular view won't be auto generated anymore. You would be able to see generated view again once you remove (or rename) your declared partial - this may be a way to make changes.
But if your system is large and you have to provide this functionality in may places you should consider extending the generator itself in my opinion. It will take longer but then you will benefit from not loosing the declarative configuration.