Many US based carriers are trying to circumvent the EU regulations and seem determined to make mechanical breakdown an "exceptional" event. This has been tested in law, it is not exceptional and they are obligated to compensate. You can file your complaint online, in English at the German Competant Authority. In Germany this is
Luftfahrt-Bundesamt (LBA)
Hermann-Blenk-Str. 26
DE - 38108 BRAUNSCHWEIG
Tel. : +49 531-2355-100
Fax : +49 531-2355-707
Email:
fluggastrechte@lba.de
You can file your complaint in English and hopefully they will help you to assert your rights.
However, the real lesson is this: When flying between the US and Europe choose a European Carrier... the only way to change US airlines behaviour and flouting of the regulations is to boycott them.