There were not many problems with her English, actually. Except the translation was so incredibly bad that at first the song was called 'Break a Leg', which they later changed to Celebrate( sounds like SALAD BREAAAAAK). It was terrible all in all.
But that was not Nina's point. I don't know if you are familiar with what happened a few days after Nina announced her song-she was attacked by a few bigoted Croatian composers for not singing in Croatian, for not hiring Croatian composers and so on. Those composers were okay with Daria singing in English and not even being Croatian(she is German).
The head of HRT also didn't give her much support, he said that we should encourage Croatian composers, but now that Nina is already chosen-we support her. That basically means: we support you, but we are not giving you any money. I would be pissed too, tbh.
Actually Daria is half Croatian and half German