C/C++ :: Determine That Char Is A Letter

I need to count how many times letter appears in a text. I know that for default letters from 'a' to 'z' and from 'A' to 'Z' there is an interval. But I need also Lithuanian letters, such as ž,č,ę. I wrote this method: (it accepts char code and checks whether that char is a letter)

bool eilutė::YraRaidė(int kodas)
{
if(kodas >= 'a' && kodas <= 'z') {
return true;

[Code]....

As you can see, a lot of checking in switch statement. I use 256 for this reason to have the same effect as unsigned char. But maybe there is a way to shrink down this method, or use some library?