@stephen, thanks for your comment! I have solved the problem! I have doenlaod the translations .po and .mo from the /dev branch and upload those 2 files to wp-content/bbpress/languages/ and rename the files to bbpress-nl_NL.po & bbpress-nl_NL.mo
@Maartje_1992, how did you use the 2.3.x translation for 2.4.1? I am also a dutch guy and i want my bbpress 2.4.1 in the Dutch langage. I have uploaded the .po file to wp-content/languages/bbpress. I have renamed the extension .po to .mo, but is doens’t working.