Ditto what Bill said.

Remember, HTTrack can't "see" anything that your browser can't.

I'm finding it difficult to think of how you would even begin to translate a
PHP site with dynamically-generated pages.  If your client is the site owner
then it should be possible to get your hands on the relevant source code,
which itself could be very tedious depending on how much of the translated
text is mixed in with the scripting.  Otherwise there's not much I think you
can do aside from over-estimating.

Sorry, but I don't think HTT is the answer for this one...