“吃软饭”用英语怎么说

没有像中文这样形象的表达。国外对这个不是特别敏感。
最接近的翻译是：connected through one's female relatives

Agree with LighsOn's opinion. In some extreme cases,

吃软饭 = male prostitute / male escort

A man who has a continuing sexual relationship with and receives financial support from a woman

eat ruan rice.

应该是吧