font.replace("ś","s");

What's wrong with this code?

1>d:\forest valley\source\intro.cpp(10) : error C2664: 'EE::Font::replace' : cannot convert parameter 1 from 'const wchar_t [2]' to 'Char'
1> There is no context in which this conversion is possible

font.replace(L'ś', L's');

What is the difference between "ś" and 'ś' ?

"" always indicates zero/NULL terminated string, which in this case translates to array of 2 characters 's', '\0'

char string[]="s";
is equal to
char string[]={'s','\0'};

's' is only one character

Ok. Thanks. Now I've got small problem, because in some texts i use txds files which are made in Gui Editor. What can I do with this texts and polish letters? font.replace makes nothing.

font.replace does work, I tested it, you need to call it after loading a font (for each font separately)
you can access all fonts from font cache 'Fonts' and FontDefault

Ok. I found a way to fix this. Thanks.