Is this translation correct? I was just looking at 吾， found in 语 so I looked for some example sentences in iciba.com and found this. I guess it's correct, but it is a bit cryptic, so I'd like to check, probably from an old book or such.

Maybe a literal translation: 莫吾知也夫! no I knows also this (I)

莫吾知也夫! Alas, no one understands me! (iciba)
莫： there is no one
吾： I，we
知： know
也： also
夫： this (person?), me

because given the English translation, I would write something different, perhaps:

唉，没有人了解我！
Unfortunately your interpretation is not quite correct.

This sentence is in ancient Chinese. 也 means nothing but to indicate the end of a phrase or sentence, and 夫 here indicates an exclamation. There were originally no punctuations in an ancient Chinese article, and these are just like punctuations. Using more than one such word (也夫) indicates stronger exclamation.

莫: no one, 吾: me, 知: understand, 也夫: exclamation, may be translated 'Alas!' 

So putting together, 

Alas, no one understands me!

or 

唉，没有人了解我！