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.