how to change the language of the theme to Dutch and how to do other translations to Dutch ?

Hi mate,

If you want to use another language for your site, pls go to Settings > General the set another language for your site http://prntscr.com/lsvtev

And if your site is just single language, we suggest you to use plugin Loco translation. It is the free plugin so you can translate manually. Otherwise, if your site is multi language, pls use plugin WPML. This is the paid plugin, however it can translate automatically your site.

Regards,
Yame