Goede middag Jos,
The translation is done centrally and changing it would change it for everybody in the (Dutch speaking) world. If you think it should be changed, feel free to join the #polyglots channel on the Dutch local slack that you can find on https://make.wordpress.org/polyglots/handbook/about/teams/local-slacks/
If you want to change it only for your own website, I would just change the width adding some extra CSS like:
.bbp-login-form label {
width: 150px;
}(you might need the !important to make it work)
Pascal.
