立食パーティーって英語でなんて言うの？

The name for a part where people stand to talk and eat depends on the situation. A formal event like this which also includes alcohol would be called a cocktail party. I believe cocktail party is the most likely answer to your question, but if the event is during the day, and particularly if there is no alcohol served, the term "reception" might be better. Have a wonderful day!

The name of a standing party where food and drink is served is called a cocktail party. Similarly, after a wedding, the 'after party' is normally referred to as a wedding reception. Not every wedding reception is a cocktail party but a cocktail party after a wedding is also a type of wedding reception.

Both describe a party where people are relaxed and help themselves to snacks or food.

These are humorous alternative answers!

You can use "cocktail party" or cocktail reception". During a cocktail party/reception, guests socialize while drinking and eating snacks.

▪ Cocktail hour This a certain time at and event or wedding reception when a time is set out for some cocktails or drinks and guest start standing around and socializing.

A buffet event is when arranged seating is not part of the proposition...People are eating and drinking while standing... "Mingling" with other guests or attendees...Usually serving themselves,from a selection of food and drinks..."I was expecting a sit down dinner with drinks...it turned out to be a buffet event instead."