I managed to access the page in English and book a practice flight to Munich from SEN; out on May 26th return on 2nd June 190 Euros return fare, not too bad. Although the return flights have different flight numbers for the morning and evenings (SIB 3010 and SIB 3030) the outward flights do share the same flight number (SIB 3000).
Now that the SEN airport website shows these routes one would expect the Munich, Osijek and Maribor airport websites to do so - they don't yet.