I have noticed that there are also other questions regarding translations and it seems there is something wrong. Since version 4 it’s not necessary anymore to use the define('WPLANG', 'nl_NL'); in your config file. Maybe that this is the problem? I can translate all the strings, but they doesn’t show up. I checked also a few scripts and the text-domain looks okay.
I’m using “Loco translate” for many themes and plugin and it works normally as it should. Any advice, I really like to help.