What are the certifications for being a linguist by occupation?

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I think the first question would be what sort of job you hope to get with your language skills. Once you have a job in mind you can look at certification for that job.

The main profession I can think of for polyglots would be translation. I don't have experience with translation certification, but a google search led me here: https://www.atanet.org/certification/ That might be a good place to start to be certified as a translator.

Another profession would be language teaching, in which case you could do TESOL certification or certification for teaching some other language.

Of course, if you really want to get into linguistics the best place to start would be a university linguistics course. Some universities offer certificate programs for people who just want to wet their feet before entering a full degree program.