I've finally managed to solve this problem by replacing all occurrences of the displayed string in lang.korean file and finding the wrong one. This was translation of "ETA".
Then I just copied the source string into translation file and after Waze client restarted, correct arrival time was displayed. The weird thing is that the same string is translated in other language files, which makes problem hunting much more difficult. Why behavior of Korean (and Latam) translations were different than other languages?
Anyways, problem is solved! Waiting for the next client update to make Korean Wazers happy