"随便" 的英文表达有很多种，常用的有:
It doesn't matter / it's all the same：没关系/都一样
It's up to you：由你决定
Whatever：随便
其中 "whatever" 这个词最常用来表示"随便"。
例句：
"I don't care what we do tonight, it's all the same to me." (今晚我不在乎我们做什么，都一样)
"Where do you want to eat? It's up to you." (你想吃哪里？由你决定)
"Do you want to watch a movie or go for a walk? Whatever." (你想看电影还是散步？随便)
"whatever" 这个词来源于中世英语 "what so ever" 意思是"无论什么"，后来缩写成 "whatever"。