Somebody who first says that he will do something, and later kind-of forgets, or assumes that people forgot. The Russian word for it is "необязательность". I guess I am looking for a translation.
It can be either an adjective, or a noun meaning such quality or condition.
Sample sentences could be:
This person is very {fill-the-word}.
{fill-the-word} has always been his weakness.
Best Answer
I would call that person a "big talker".
The term refers to somebody who is "all talk and no action".
That term refers to somebody who makes lots of commitments, but doesn't follow through.
Depending on your context, that person can also be described as simply "unreliable".