How do European PhD interviews work when a candidate is not local?
Interviews through teleconferencing (e.g. Skype) are pretty common if it is not feasible to invite all shortlisted candidates for an in person interview. 

However, since we are talking about Slovenia to Germany, it is very well possible they would would invite you to come over if they wanted to interview you. In such a situation it would be standard practice to reimburse your travel costs.