Hello,
if someone faces this issue, seems like this was an answer:
// define('LANG', 'en_GB'); // default fallback language define('LANG', 'en_US.UTF-8'); // default fallback languagein webapp/config.php
Do not mess with default settings… :) Well, I had it set to et_EE-UTF-8 for me and it was working for a looong time.