While EmulationStation is used as a part of RetroPie, it is not written or developed by RetroPie’s devs.
Sadly, the original developer of ES has been out of the picture for the last few months. That means that very little is currently possible within the ES software.
In it’s current shape there is not even full unicode support let alone a proper localisation layer to enable different languages.
If you feel that having ES in your own tongue is essential for your needs , I would suggest looking at Recalbox, which is also based on ES, but have localisation options (i.e. Many different languages). They support fewer systems though.
Thanks for your reply, I mean I don’t bother having ES in English, just would be (even) better and more customizable.