Translation CoursePress to dutch

I like to translate CoursePress to Dutch though it is not working after making these steps:
1. Downloaded .po file via
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,