The City of Birmingham Symphony Orchestra under Simon Rattle is the resident orchestra Aug. 22 to 26, and among the other polyglot offerings are ''L'An Un,'' an opera-ballet on French revolutionary themes by Duncan Youngerman, choreographed by Andrew Degroat, Aug. 25 and 26 and the Moscow Taganka Theater's production of Pushkin's ''Boris Godunov'' Aug. 22, 23, 25 and 26, in Russian with simultaneous translation. There are some tickets - none more than $28.80 (prices at 1.6 pounds to the dollar) - for most events for last-minute customers, available from the porters at the major hotels, or from the festival ticket office (21 Market Street, near Waverley Station; 226-4001), open Monday to Saturday from 10 A.M. to 6 P.M., and also from 10 A.M. to 5 P.M. Sunday during the festival. An information booth nearby, in the parking lot of the National Galleries of Scotland, sells some same-day tickets for half price, no more than two to a customer, from 1 to 5 P.M.