Translation CoursePress to dutch

Hi,

I like to translate CoursePress to Dutch though it is not working after making these steps:

1. Downloaded .po file via https://premium.wpmudev.org/translate/projects/cp/nl/default/

2. Uploaded it into languages of the plugins and also the individual plugin languages directory

3. Added define (‘WPLANG’, ‘nl_NL’); in wp-config

4. reinstalled the core to Dutch (even though this was already in Dutch).

See no difference yet. You know what to do extra?

Thanks in advance,

marjet