Nowadays, most of text data are stored in UTF-8 format. If the language uses UTF-8 as native string format, it is much easier for text processing, which is why Go chose it. There are a lot of subtle technical details you will only realize after working with many text processing components.
I don't understand where you see the mistake in Python 3 with unicode. What encoding Python internally uses to store strings doesn't really matter. What's important is that it is always known what encoding is used. This was unclear in Python 2 and Python 3 fixed this.