Here an another X/Unix problem arises. IFAIK, You cannot type chars that are not supported by Your current locale.
It seems to be a trend in current distros to move to UTF8-based locales. Those support everything you throw at them. On my system (FC6), it's like so: $ echo $LANG en_US.UTF-8
With this locale setting I can type any language I set my keyboard to. Most applications handle it gracefully (Qt- and GTK- based ones do, at least).
I don't think that there's any way to go about those things but to use UTF8-encoded locale. Since UTF8 is just the encoding, the actual locale (language and locality) can be set separately. You could have by_BY.UTF-8, or even ru_BY.UTF-8.
Cheers, Kuba