I've encountered problem with non-latin characters too.
I've tried to run xpdf.bin direcrty, and find out that problem doesn't
disappear.
I do not have such problems with other Motif applications. For instance,
I use vim with Motif GUI on daily basis. There are some problems with
dialogs (which seems to be gvim problems rather than Motif problems
because Xaw GUI of vim suffers of them too)
I've grepped xpdf sources for setlocale or
XSetLocaleModifiers system calls and found none.
So, it is understadable why xpdf doesn't input non-latin1 characters -
XLookupString (and XmbLookupString) properly converts keysym into
character code only if locale is set correctly.
I've tried to add setlocale(LC_CTYPE,"") at the begining of main
function of xpdf, and this have allowed me to enter cyrillic letters
into find dialog, but these letters was displayed incorrectly with some
Latin1 font, even if I've explicitely specified cyrillic font with -fn option.
I think that if upstream author is SO unaware about handling of
non-latin1 characters in X, there should be a bit more work required to
turn entered search string into correct representation for search inside
PDF.