Hello Tobias,
Yes, there are technical reasons for using asian UTF-8 characters. A UTF-8 character can hold up to 4 bytes of data while a "regular" ASCII character can only contain 1 byte of data. So by using these UTF-8 characters Boxcryptor can store more data in one character - this means shorter encrypted filenames. This is especially important on Windows where many programs have maximum path length of 255 characters.
Best regards,
Robert