You may have to do some quick research on terms that would be unique to a diversity candidate and build up a list. Candidates probably would not put the word "diversity" on their resume. For example, a candidate may list "Pi Alpha Phi" on their resume which is a Chinese Fraternity. A candidate may also list professional organizations like the NBMBAA on their resume which is an african american MBA conference.
You could try adding something like this to your string although this search won't be 100% accurate.
"(fluent OR bilingual) AND (spanish OR cantonese)"
When you are a diverse professional engineer, most likely you belong to a professional organization. For example, if I am a Hispanic Engineer, most likely I belong to SHPE, Society of Hispanic Professional Engineers. Google: diverse engineering organizations or associations to find others and their acronyms. Then do your normal search. Also, find out where they hang out: what churches, restaurants, etc. they go to...find them.