In computers, a
bit is the base of any data. For practical reasons, a
group of 8 bits is used, the name is
byte.
A
byte is the
minimum unit of data available. Absolutely every thing is made of
bytes.
All you need is to cast anything to a byte array and feed the encryption routine with blocs of 8 bytes, no matter what they are. There is no interpreting to do, English letters or not.
Encrypted_data = IDEA_Encrypt(Original_Data, Key);
Decrypted_data = IDEA_Decrypt(Encrypted_Data, Key);