Never going to happen because Steam would have to pay for a translation service which are never cheap.

There's a transactional difference between Steam offering a translation button redirecting to Google Translate and you copying said content manually to Google Translate. The difference is 20 USD per million words it would cost Valve to provide this function.

If 1% of the concurrent users (400k users of 40M) would translate the above paragraph (39 "words"), it would cost Valve 312 USD. If they cache the result and the hitrate is 97%, it would still cost Valve 9.36 USD. Just for these 39 words alone.