bite off more than one can chew

 لقمه بزرگتر از دهان برداشتن

to take on more than one is capable of; to take on too much

EXAMPLE 1: Jennifer is having a dinner party for 50 people, and she can't even cook. I think she's bitten off more than she can chew.

EXAMPLE 2: You agreed to host 50 exchange students from Korea? Aren't you afraid you've bitten off more than you can chew?