。
I've never eaten a mango.
Literal
I [topic-は] still mango [object-を] eaten-[experience-ことがない].
~たことがない ('have never done') is the negative of ~たことがある. マンゴー is an English loanword. まだ ('still, yet') adds temporal nuance — 'not yet, so far.'
。
I've never eaten a mango.