MIB Designer supports UTF character sets, because it is written in Java. You can enable UTF-8, for example, by running MIB Designer with
java -Dfile.encoding=UTF-8 -jar mibdesigner.jar
However, SMI does only allow 7bit-ASCII characters and
- carriage-return (CR)
- line-feed (LF)
- tabulators (e.g. TAB)
- spaces
for all text enclosed in double quotes (") within a MIB file.