The Tchibo stick is netlocked, so it can be used only with German O2 SIM cards. But unlocking has become very easy: Just calculate your individual unlock code at
Online Huawei Modem Unlock Calculator, insert a SIM card from any other operator in your stick and start Mobile Partner. After requesting the PIN code for the SIM card, Mobile Partner will ask you to enter the unlock code, which will unlock your modem permanently.
Afterwards you'll just need to create a connection profile for your new provider as described at
Huawei Mobile Partner - 3G modem wiki.
And yes, you can use any Italian or Croatian SIM in your Tchibo stick, as it fully supports all Italian an Croation networks. I think there are threads about prepaid data offers for both countries somewhere on this board.