Widi: We did this purposely. We did not copy the polygons and flipped their normals for the leaves. This can produce some other problems (z-fighting) and creates more polygons. But you can do it yourself.

I checked the models in different real-time viewers and in a game engine, as you can see here (click right mouse on image to display it in higher resolution):



As you can see, the transparency and sorting works perfectly with the right shader. Even shadows work great (shadow mapping, stencils will not work) and shadows can take the alpha channel into account. So you can also see, that our alpha channels are well made.

In that engine I only had to set a checker for "double sided rendering" and for "alpha testing". I applied this material only to the leaves. The bark has another material with a normal map for the bumps.

I know, that this can be done in Gamestudio as well, we did it once with our games (Dexon Knight as an example). But you need to make a custom material for that.