str_printf indeed supports no Unicode yet. That beta feature was a little premature - the next version will support Unicode for str_printf, but not the current version.

Here are all flag definitions for strings:

const long STRF_CHANGED = (1<<0); // String was modified
const long STRF_NOWIO = (1<<1); // String is edited with INKEY
const long STRF_SYS = (1<<2); // system string / don't deallocate
const long STRF_NCRLF = (1<<3); // Crlf Count yet to be done
const long STRF_NSAVE = (1<<5); // Don't save this string
const long STRF_UNICODE = (1<<6); // String contains 16-bit characters
const long STRF_NOHASH = (1<<7); // Ignore '#'