When printing message with special characters Unity doesn't display the message in Editor-console, but you can see the message in Console (see the screenshot).

The string is parsed from fbx file. The string is "D:\Props\Pictures\peñasco.tga" (i.e. problem is in letter n with ~ on top of it). In debugger the symbol is displayed as '\xf1'.

Doing this:

Debug.Log("---ñ---");
Debug.Log("---\xf1---");

from script works fine, so it must be something wrong with conversion from C++ to Mono strings.