There is no outright reason why either Sulu or Valeris
should be aware of where this top secret conference is going to be held. Supposedly, all three top schemers (Cartwright, Chang, Nanclus) would be involved in the conference and aware of its location - but Valeris would be out of the loop, isolated from her secret superiors ever since the ship left spacedock, and thus unable to contact them for details once the conference location was changed from Earth to Khitomer.
On the other hand, Kirk is out of the loop, too. Perhaps everybody else in Starfleet already automatically knows this "secret" location, what with being part of organizing the event and all? But Kirk is no longer in Starfleet, not in practice. If he just contacted home base, the response to his question would be "Thank you for calling, now stand by to be arrested".
So his best choice is to ask the resident traitor; his second best choice is to ask a Starfleet member he trusts not to betray him. Whether either of these actually is a likely keeper of the secret is sort of unimportant.
As for the translation scene, the novelization IIRC offers the obvious explanation: Valeris, already known to have sabotaged the torpedo count computers, would have sabotaged the translator program as well, as a precaution to hinder Kirk in his possible attempts at contacting the Klingons and exposing the plot to Azetbuhr.
The movie claims that use of the translator program would be "recognized" but doesn't explain how, so we're ready to invent our own reasons for why our heroes use physical books rather than computer databases.
The penultimate script version, found at TrekCore (with Saavik still the traitor), omits all explanations as to why books are used, making it even easier to dream up explanations. Spock cutely just asks Uhura for "a feat of linguistic legerdemain", and the books apparently are it. Perhaps Starfleet just plain doesn't have translation programs for Klingonaase at this point of history: every encounter with Klingons involves the adversaries speaking English to their counterparts of captives, and other sources to the language are difficult to come by. So, all the Feds have is fragments and speculation, bound in books by competing scholars with competing ideas, and never amounting to a reliable translation program. The clumsy "subtitling" in ST:TMP might be a state of the art best guess, and might be completely incorrect...