I tried deleting system and author translations leaving only the custom one. It’s still working.

On my child theme instead, I have to keep translations in custom folder, if I leave files only in author or system it does not work(I’m loading with the load_child_theme_textdomain function).

I’m a bit confused, but the important thing is that we can present the website in the right language.

So, many thanks for your commitment @timwhitlock