First of all, did you copy the extra Dutch language files for bbPress and BuddyPress? You need those in addition to the Dutch language files for WordPress itself.
https://codex.bbpress.org/bbpress-in-your-language/ will point you in the right direction for making things work with bbPress. (I suggest that you take the “dev branch” for the files, as the other link is outdated.)
https://translate.wordpress.org/projects/buddypress/dev points to the BuddyPress version of the language files.
If you’ve got those up and working, you should be able to use the Dutch settings… Although I don’t know if the plugin will enable you to switch languages for the bb’s, as I can’t find that they support them.
In ieder geval veel succes!
I have bbPress working in Dutch now – including switching languages.
In fact, replies post in one language will not show-up in another.
So all-and-all it works great sofar!
Now the theme formatting… but that is a different story => different topic. 🙂
