I have the same problem suddenly appearing after several months Joomfish was working perfectly fine.
Its strange. Sometimes the menu translation is working and sometimes not. It is also happening that the translation changes from german to english randomly without doing anything. I cannot explain that and i didn't find anything.
This problem i have on two sites:
http://winnieguesthouse.com - Joomla Version 1.5.26 / Joomfish Version 2.1.7
http://www.info-namibia.com - Joomla Version 1.5.23 / Joomfish Version 2.1.7
Any help yould be highly appreciated.