Maxima allows Unicode, if it was compiled using a Lisp compiler that either supports Unicode or that doesn’t care about the font encoding. As at least one of this pair of conditions is likely to be true.

wxMaxima provides a method of entering symbolic characters using the keyboard.

Unicode

Computers traditionally stored characters in 8-bit values. This allows for a maximum of 256 different characters. All letters, numbers, and control symbols (end of transmission, end of string, lines and edges for drawing rectangles for menus etc.) of nearly any given language can fit within that limit.

For most countries the codepage of 256 characters that has been chosen does not include things like Greek letters, though, that are frequently used in mathematics. To overcome this type of limitation Unicode has been invented: An encoding that makes English text work like normal, but to use much more than 256 characters.