Greek Small Letter Pi "π" is:
Unicode character: 0x03c0
cp437 encoding: 0xe3
The character isn't part of Windows-1252.
The Windows-1252 character with encoding 0xe3 is Latin Small Letter A With Tilde "ã" (unicode 0x00e3). So, if you were to open a CP437 plain text file in Notepad, "π" would show up as "ã". (Because Notepad assumes Windows-1252 for non-UTF files.) Were you then to copy it from Notepad and paste it to the forum, it wwould be the wrong character, "ã". Were you to copy it from Notepad and paste it into Qbasic, it would be a normal lowercase "a" (ascii 0x61).
I think it would be better to restrict the programming language to characters that are encoded the same in CP437, Windows-1252, and Unicode.