Do you speak the langauges??
I know there are cards that many print out off online, that will state a persons allergy in whatever language they choose,, very helpful to hand to servers etc. but, you still have to be so careful,, as many people do not GET IT.. they figure that its ok to chop the nuts ont he cutting board,, then slice your bread on same board,, see what I mean?
I was on a Family Tour with a lady whos daughter had life threatning peanut allergies. Mom had dd wear a pack with three epi pens in it at all times. She also always always double checked with kitchen. She forbade icecream as she said they often use same scoop in all ice creams,, including the nutty ones. She brought an entire bag with peanut free chocolates and granola bars etc so her child could have treats.
We did not travel to Croatia or Slovenia,, but I imagine you must take similar steps. Most staff this lady encountered were kind,, but really, not that aware,, so it will be up to you to be blunt in what is ok .
Good luck and have fun.