But it turned out that ECL supports multithreading.
Yesterday I downloaded, build it and tried simple test: (mp:process-run-function 'zu (lambda () (loop (format t "zu~%") (sleep 3))))
Seems working.
Also ECL may be build with Unicode support. Although it doesn't support external formats in streams, strings and characters are represented as 24-bit values. It should be sufficient to have support for various external formats in IO using flexi-streams.
Good news for me. I pin great hopes on ECL.
P.S.
I discovered ECL via Common Lisp Survey linked from the xach's post. Thanks ... all.