If you use a Polish font and U_ASCII, then you basically limit your output to English (standard 7bit ASCII) and Polish with a few other languages that use the same extended ASCII layout as Polish. The prefered way is to use Unicode (the default mode which is U_UTF8 should work just fine) with a Unicode font. But then all your strings must be written in UTF8. This can be a bit tedious if you hardcode your strings in C/C++ code, because you aren't supposed to encode C/C++ source files in UTF8, so you have to use escape sequences (or whatever they're called), like in the Allegro's Unicode example.
A better way is of course to write all your game strings in a separate UTF8 encoded text file, which you load in your game at start up. If you do this, it is also very easy to add support for other languages to your game. You just write a text file with strings for each language and load only one of them at runtime.