Loads of jobs at the U.S. Department of State do not require a college degree at all, though they get enough applicants with degrees that you need lots of experience to compete. Specifically, no degree is required to take the FSOT.

Oddly enough, the jobs which do require a college degree (things like IT) often do not require foreign language fluency for overseas postings. But then there's the question if those are technically "diplomats", as they are likely defined as "administrative and technical staff" rather than "diplomatic staff" under the Vienna convention.