FLTK is permanently stuck in the 90s. It doesn't have full unicode support, right-to-left & bidirectional text, and doesn't support accessibility tools.
Not to mention that development has been stagnant for 15 years since it's flagship application (Nuke), was ported to QT.
Actually FLTK has full unicode support, right-to-left and bidi text.
The limiting factor is the OS.
For rtl and bidi, it depends on the OS. On linux for example, if FLTK is built with pango support, it'll support them just fine. On windows and macos, these work out of the box.
As for accessibility, the only thing missing is screen reader support. Otherwise tab navigation, ime and other modalities are supported.
FLTK's bindings in Rust provide an accesskit adapter which supports screen readers using the accesskit crate.
Not to mention that development has been stagnant for 15 years since it's flagship application (Nuke), was ported to QT.