Breadth is nothing but Melbourne's fancy name for electives, albeit "electives" they force you to take.
The monash BSc is an incredibly flexible degree. As long as you satisfy all the requirements in the handbook (Eg. You actually do some science...including a major, minor and few other reqs) you can take whatever unit you like. I've studied 3 philosophy units, a history unit and an astrobiology unit all as "electives".
Unlike some of the more restrictive degrees, long as you do what they want you to do, you're totally free to use the rest of the room available to pick whatever you like.
You can achieve the equivalent of a major in a language using the spare subjects in your degree (aka electives). I dont know the specifics but i think this would use up either all or almost all of your electives but it's doable. If you want to keep your electives open and take a language, the diploma is an option.