For good advice, the departure times are also necessary. That can make quit a difference and we cannot guess which trains you will be taking.

Berlin - Krakow: 10.39 hrs, 12 July direct. 
Budapest via Zagreb to Split: 15.12 hrs, 19 July to Zagreb and then Zagreb to Split 01.10 hrs, 20 July. 
Split - Ljubljana via Zagreb: 14.25 hrs, 26 July to Zagreb and then Zagreb to Ljubljana 21.27 hrs. 
 

thanks so much! 

Berlin Hbf - Krakow 10:39 can be done at DB ticket office.

Budapest Deli - Zagreb 15:35: reservation is optional, DB should be able to do that.

Zagreb - Split 1:10: reservation (for seats) is optional. Probably only possible in Croatia but you can try DB.

Split - Zagreb 14:25: might be possible via DB but is much cheaper in Croatia (~1€). You could book on arrival in Split on 20 July.

Zagreb - Ljubljana 21:27: DB should be able to do that.

By the way, there is also a direct night train Budapest Keleti 18:45 - Split 8:12. I don't know if DB can book it, but you can book it online via MÁV (add "Bérlet (pl.InterRail)” as discount); reservations start from 30€ (couchette, no seats in this train as far as I can see).

Thanks a lot for your advice. That helps so much and saves a lot of stress

Schon pass gekauft?

Its seems you are from B? FRankly, knowing the ´normal=low´ prices for zuge over there you would be wise to check if just booking normal tickets (and it seems you want those days only) will not work put cheaper. You then have to book on site sof railway there train starts.

The service to SPl is very much reduced and about the only remaining railline to coast-normal Croats go by bus (ev 30 mins)-expect overcrowding (but that will also occur in DE with this 9€)