Dutch translation not translating all English text

Some of the text is not being translated, even some of which i seem to be unable to find in the code of the plugin either.

ex. after creating an account, you get a message that reads something like “Congratulations for your account creation”, where are these texts in the plugin? I can’t find them anywhere.

Any help on the matter would be much appreciated.

https://wordpress.org/plugins/wp-members/