What do you mean ‘Croatia is not even a directly neighboring country anymore’?

Last I checked, Croatia and Hungary still share almost 350 km of border.

Pirate Party in Slovenia did not qualify for elections. They needed the support of 1.000 constituents, but when the officials did the count they only had 970 signatures.

What I’m saying is - your mileage my vary.

In all seriousness, I think I’ve seen ‘Mx’ used in this context.

Usually though, you’d go with a generic greeting such as ‘Dear Sir or Madam’ or ‘To whom it may concern’.