How is this sentence translated into Chinese "The man, who kissed the woman, who betrayed him with the man, who loved the one, whom nobody knew"?
The words 'who' in this sentence is called 'relative pronoun', the clause introduced by which is called 'relative clause'.

A sentence with more than one relative clause is said to contain 'nested relative clauses'. The nesting structure in your sentence is:


  The man, who kissed (the woman, who betrayed him with (the man, who loved (the one, whom nobody knew))).


There is no relative pronoun or relative clauses in mandarin Chinese. There are three common methods to translate such sentences:


Turn each relative clause into a sentence. There will be no more nesting. The sentences will be longer but very clear.
Turn each relative clause into an adjective phrase. There will still be a nesting structure. The sentence is shorter but the nested adjective phrases may be ambiguous. 
A combination of #1 and #2 - use #2 for clear and short fragments for brevity and #1 for the rest of the sentence for clarity.


Translation using method 1:


  男人亲吻了女人，女人却为了另外的男人背叛了他。那另外的男人也爱上了别的人，是谁却无人知晓。


Translation using method 2:


  男人亲吻了那为了一个爱上了无人知道是谁的人的男人而背叛了他的女人。


(This sentence is practically unreadable but looks fun.)

Translation using method 3 (combination of method 1 &amp; 2):


  男人亲吻了那为了别的男人而背叛了他的女人。那个另外的男人却也爱上了别的人，是谁却无人知晓。