[vkreso]

An aria from a Croatian opera, "U boj, u boj", is sung by Japanese choirs since WW1. It was first thought to be Czech though because Czech sailors played it while repairing their ship ashore in Kobe after the great war.

https://en.m.wikipedia.org/wiki/U_boj,_u_boj

It is sung in Croatian!

https://youtu.be/uqQf_92dgHM