What was the language used in the piece "Tales of The Future"?

Reportedly, it's not in any actual existing language; rather, singer Demis Roussos used sounds that, uhm, sound vaguely Middle-Eastern. [If anyone knows a better way of formulating that, do let me know]

However, here and there actual Arabic words may have slipped in.

What is the music Rachael is playing when she is in Deckard's apartment?

For this scene, Vangelis composed a variation on Chopin's "Thirteenth Nocturne". (Trivia: the "temp" music for the scene was a variation on Ravel's "Piano Concerto".)

What is the music heard when Roy & Leon are on their way to visit Chew?

The music is from "Harps of The Ancient Temples" by Gail Laughton.
The album is listed as cat #111 in the old CD catalogs on the Laurel label.

What is the Japanese music that the blimp flying over Sebastian's apartment is playing?

The old BR FAQ by Murray Chapman provides the following information:

"Japan: Traditional Vocal and Instrumental Music, Shakuhachi,
Biwa, Koto, Shamisen"; performed by Ensemble Nipponia, 1976.
Electra Asylum Nonesuch Records/Warner Communications Inc.
(Compact disc; catalog number not available)

The music is produced by plucking on what is called a "biwa" - a traditional Japanese instrument.
The Japanese lyrics are from a song called "Ogi no Mato", which is, according to FN, "part of a song cycle/epic somewhat reminiscent of Shakespeare's 'Romeo and Juliet' and the William Tell legend (as 'Ogi no Mato' features young people from opposing clans and a crucial moment of archery)." Reportedly, "the lyrics tell of the tragic and utter destruction of one Japanese clan by another."

What is the music that is playing in Taffey's bar when Deckard enters?

Since that track is not on any of the soundtrack albums (not even on the Off-World and Bongo bootlegs), this has been some kind of a mystery, but most likely it's simply one of those many little Vangelis gems that never made it to the soundtrack album. The vocals are definitely done by Demis Roussos.

In the BR Workprint, an instrumental version of "Qu'ran" by David Byrne and Brian Eno from "My Life in the Bush of Ghosts" was used in Taffey's bar.

What is the music playing when Zhora is "taking the pleasures from the serpent"?

When Zhora starts her "act", a disco-like music starts playing; like the previous track, it is not on any of the soundtrack albums. Probably another short Vangelis mystery track, but who knows?