Protein translation does take place in mammalian sperm prior to fertilization.

Researchers demonstrate that human, rat, bovine and mouse sperm all incorporate labeled amino acids into polypeptides during the capacitation phase. They identify that mitochondrial translation machinery (as opposed to cytoplasmic) directs translation of nuclear-encoded genes in sperm, and that its inhibition leads to a marked decrease in sperm motility, actin polymerization, the acrosome reaction and in vitro fertilization rates.

Thus, protein translation in sperm is essential for sperm functions that directly contribute to fertilization. The new findings would give us better understanding for treatment of male infertility and developing new male or female contraceptives.

How can mitocondrial translation of nuclear encoded genes occur ? Mitochondria have their own DNA and ribosomes. What is the role of translation of nuclear DNA in sperms then ?
Please clarify.

To answer your first question, mitochondrial translation of nuclear DNA probably occurs through mRNA signaling to the mitochondria after it is spliced and edited. The targeting sequences are usually found in the 3' or 5' untranslated regions.

As for your second question, in the primary article (the link is below) Gur and Breitbart hypothesize "that new proteins are synthesized in spermatozoa to (1) replace proteins that degrade in the delay before fertilization and (2) produce capacitation-specific proteins".

And here's the link for the primary article: http://www.genesdev.org/cgi/content/full/20/4/411