Is what I did unlawful (haram)? And what is the expiation (kaffarah) for eating chicken not slaughtered Islamically in a European country, especially when one forgets and no Islamic alternative is available?

Going to non-Muslim countries exposes a Muslim to doubts. A Muslim must stay away from forbidden things, and it is preferable to stay away from doubtful things as a matter of piety. Hunger is not an excuse to eat meat that has not been slaughtered according to Islamic rites, as alternatives exist. European meats are permissible if slaughtered according to Islamic law by a Muslim or a person of the Book, and forbidden if killed by electric shock or strangulation, or if the slaughterer is not a Muslim or a person of the Book. Negligence in asking about the ruling on meat, if due to forgetfulness, is forgiven, for the Prophet, peace and blessings be upon him, said: "Indeed, Allah has forgiven my Ummah for their mistakes, their forgetfulness, and what they are forced to do."