L2J Server

I'd like to make shop where i give 20 Alchmist Chest Key(ID=9205) and in return get RANDOMLY one Interlude weapon(ID's=8678:8679;8680;8681;8682;8683;8684;8685;8686;8687;8688)

I think it would be best way to do that with adding some kind of new quest...but im not so good at writing that with random givings.

I'd be glad if someone could help me

I think is not DP side...i think...java...or jhyton(?)

Yes you may be able to create this as a jython quest script.

If you can think of another quest that gives you random rewards have a look at it's quest script to try and write something for yourself.

You can definitely achieve this with Jython. However, there is also another way with a multisell and a custom extractable item (it assumes that you can find an existing junk item supported by the client and then modify it server-side to make it extractable). For more details on this solution, see here:
http://www.l2jdp.com/forum/viewtopic.php?t=9013