Hi,
I've found the culprit. I had synchronisation of 'custom fields' in Polylang settings activated. Therefore it also synchronised the text between languages (which seemingly uses custom fields). By deactivating it and entering the translated text again everything got solved.
Not sure if not synchronising 'custom fileds' will bring me into trouble om some other part of Wordpress, but for now everything is back to normal.
Thanks for your quick response. The added features for version 1 are really great.
Jan