永不满足的英语怎么说

Insatiable

形容词 a.

1.永不满足的;贪得无厌的[(+for/of)]
They are insatiable of profits.
他们一味追求利润。
He had an insatiable love for music.
他对音乐爱得入迷。

词义来讲最简单直接的是形容词Insatiable即可

但短语一般还是要翻译成短语形式，比如
Never Full
Never Enough

你要完整点的结构还可以说Somebody never feels complacent.