[chrishajer]

The define statement should look like this:

define( 'BB_LANG', 'nl_NL' );

if your file is named nl_NL.mo in the my-languages/ directory

The format for the name of that file is

language code_underscore_country code.mo so nl_NL.mo and in the config you drop the .mo

More info about Dutch for bbPress here: https://bbpress.org/forums/topic/dutch-translation#post-56700