“自作主张”可以翻译为“act on one's own initiative” 、“take the liberty of deciding（或acting） on one's own” 或者 “make decisions without consulting others” 。
“act on one's own initiative”强调主动、自发地采取行动，不依赖他人指示，突出自主性。例如：He always acts on his own initiative, never waiting for others to tell him what to do.（他总是自作主张，从不等着别人告诉他该做什么。）
“take the liberty of deciding（或acting） on one's own”侧重于表达在未经他人允许或未与他人商议的情况下，擅自做出决定或采取行动，带有一定越权的意味。例如：She took the liberty of deciding on the project without consulting the rest of the team.（她自作主张地决定了这个项目，没有和团队其他成员商议。）
“make decisions without consulting others”意思是没有与他人商量就做决定，更直白地表达出这种擅自决策的行为。例如：It's not good to make decisions without consulting others.（自作主张做决定可不好。 ）