I think this component is intended to do what you want: https://www.softlab24.com/product/view/37/user-categories

Please, I need a dropdown field code to add on my signup page. I want to add another drop down field for users to select there "Departments" which is ; there work field. For example, Web developer, Graphics Designer, Programmer and lot's more. Also I want it to display the user Department on there about page when users click to view there about profile.
Please I'm looking forward for help.
Thanks In Advance!

@claus
please start a new thread either here or at Github regarding this language file strangeness. I can try to investigate what's special with this file then.

Waoh!!
I could say I'm most grateful!
I have resolved it perfectly. I followed all guidelines and it all works. My registeration page have now been the way I wanted it.
Much thanks to @Claus Lambert & @Zet Man for there help so far.

IAppreciate

@Zet Man: You can believe me: everytime I change the file /locale/ossn.de.php there is this error! At this moment I changed the phrase "Email (nochmal)" to "Email (nochmals)" just to simulate it and after saving, my site stays completely white! It´s only this file! I have translated some other ossn.de.php - language files, for example the forum-component, and I never have a problem with the other files.
Meanwhile I use the component "Custom Strings" when I need to change a phrase originally in /locale/ossn.de.php

First, there is a little error in my code, I wrote "passwort" instead of "password":

File: /system/plugins/default/forms/signup.php
Line 47 add:

if ($user["repassword"] !== $user["password"]) {
    $em["dataerr"] = ossn_print("password:error:matching");
    echo json_encode($em);
    exit;
}

To remove the mail-field:

File: /system/plugins/default/forms/signup.php
Line 19 delete:

$user['reemail'] = input('email_re');

File: /system/plugins/default/forms/signup.php
delete line 19:

<input name="email_re" type="text" placeholder="<?php echo ossn_print('email:again'); ?>"/>

Delete this section starting at line 41:

if ($user['reemail'] !== $user['email']) {
    $em['dataerr'] = ossn_print('email:error:matching');
    echo json_encode($em);
    exit;
}

Hope it works for You

I don't wanna sound destructive at this point, but to me all this doesn't make too much sense. :)

The complete password recovery mechanism is based on a 100% verified and correct email-address.
If you have a typo in your email and you don't remember your password, there'll be no way to reset it, because OSSN is asking for the your email-address.

@Claus regarding language file editing: Simply turn on error reporting before editing the file. You'll get a reliable and helpful message including the line number pointing to the location where you fucked it up.

I tried removing the re-enter field from forms/signup.php
But it stops working on user registration !

Thanks Claus. For a wonderful and well written instrustions.

I have done it the password is now working. But I totally mean to remove the re-enter email field.
I want to use re-enter password to replace it. So there is no need for the re-enter email field again. I only want one input email field. At least that's okay to me.
Please how can I remove that? Entirely.??

And remember: NEVER EVER edit a core-file! Do it on Your own risk!