I live in small country named Croatia. Like most of countries on "old continent" we use some specific letters. For Croatia those specific letters include Č, Ć, Đ, Š and Ž. Usual way of entering those letters is to use Symbols screen and select from all special characters. It is not ideal, but it is manageable.
Only problem is letter Đ. For some reason, Microsoft decided to omit that one from symbol selection. Don't get me wrong - that letter is supported - problem is how to enter it on mobile device.
To solve it, I just do entering of this letter on computer and let it synchronize to mobile. If I really need to enter it on machine, I just open some old contact with that letter and copy-paste it in correct place.
Only issue I had with that plan was that Windows Mobile 6.0 Standard (Smartphone) has no copy-paste support out-of-box. However, there is nice freeware program called VITO CopyPaste that works pretty good.