English Original Sentence
She lacks confidence.
I read the English as “she’s not very self-confident”. If that’s true, does the Italian convey the same meaning? (I’m not a native English speaker.)
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I also read the English sentence as you do.
The Italian sentence is a bit ambiguous, it may work as a translation of that.
I would use either “Le manca la fiducia in se stessa.” or “Non ha autostima”. The latter is actually “she has no self-esteem”, but I think it works as a synonymous.
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I am also not a native speaker, but I think there is a difference.
I may totally lack confidence in certain situations - e.g. while dancing or talking in Italian 
- but that does not harm my self-esteem, unjustly or not.
Therefore it seems to me that without context the sentence is ambiguous in both languages.
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I am a native speaker! 
Yes, I agree, without context it is ambiguous in English too.
It could, as you say, mean that she generally has high self esteem, but lacks faith in her ability to ride a unicycle without falling off.
That said, in the absence of any context, I would probably read it as saying that she lacks (self) confidence generally.
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