2.8: How to texture paint some elements of mesh only? I'd like to know if there is a way to paint some parts, in my case the eyes of my model, without touching any other part. 

I've tried to in Edit Mode select the UV Sphere, that is already joined with the body of my mesh, pressing CTRL + L and hide everything else. 



But as soon as I change to Texture paint mode all my mesh appears again and when I paint I paint not only over the eyes but the hole body...
You can make use of the Face Select Mask setting to limit the effect of your texture paint to selected faces in Texture paint mode.
First image is the basic setup in Texture Paint.

Second image is turning on the Face Select Mask and making a selection on the model.

Third image shows a result with two areas painted, one selection active.