I'm going to travel in six different European countries for about 2 months.
I want to buy a sim card, which could be a data-only sim card.
Should I buy a local sim card in each country, or is there a European sim card that can save me money?
Since 15th June there are no additional roaming costs. So you can use one card in all countries.
When? Now, best to get a SIM for each country, after mid June you *should* be able to roam, but mind the small print.
Croatia is part of the EU, where from June 2017 new international roaming rules are enforced. In most European countries you can now 'roam like at home' at domestic rates rather than excessive roaming rates. Croatian providers implement the principle without restrictions. Only unlimited packages are capped.