Your choice of X toolkit will be connected to your choice of
application language in two ways: first, because some languages ship
with a binding to a preferred toolkit, and second because some
toolkits only have bindings to a limited set of languages.
The once-ubiquitous Motif toolkit is effectively dead. It
couldn't keep up with the newer toolkits distributed without license fees
or restrictions. These attracted more developer effort until they
surged past closed-source toolkits in capability and features;
nowadays, the competition is all in open source.
The four toolkits to consider seriously in 2003 are Tk,
GTK, Qt, and wxWindows, with GTK and Qt being the clear front runners.
All four have ports on MacOS and Windows, so any choice will give you
the capability to do cross-platform development.