Are there proficiency tests that someone can list on their resume to prove their ability in a certain (natural) language? And if so, is it worth getting these certifications? As in, will they give you an edge over other applicants?
Google searches turned up a bunch of language interpreter or language teacher links, but I'm interested in just listing that I know such and such language for any kind of job.
Yes, there are! Many institutions release them, for example the Goethe institute or The Académie Française, and many other depending on the language. In Europe specifically, there is also a language-independent standard, the Common European Framework of Reference for Languages (CEFRL). It defines multiple "reference levels" of language competence, which you can get certificates for.
They are a proof of your knowledge, and can be used to enroll in universities, to apply for citizenships (not in every county), and in some jobs.
They do help if you work in certain fields, like multilingual customer service; however, often the companies will also (or instead) check directly your knowledge of the language before hiring you (usually by making interviews in the specific language).
Finally, they are not free and depending on the level you want to certify, the preparation needed may be demanding. So if you plan on taking an exam, try to research first what level will be useful or required for you - for example, with CEFRL, when one is required for a job, it will usually be at B2 or even C1, so getting a certificate at A2 will be of limited use.
In Europe you can often find jobs, degree programmes, and government applications (eg. citizenship) where an official language certification is required. They appear to prefer two particular certifiers which as far as I know are commercial ones (IELTS and TOEFL).
As mentioned, they are usually a requirement, so having them is not just beneficial.
As for languages in general, I don't think it is that useful unless your interviewer or manager is originally from that country. Then it might give you a slight edge over other equally skilled/experienced candidates.
Yes, taking a language course will provide you with some sort of certificate if you pass usually, and in many places it can be a requirement and is much better than nothing.
Many jobs in my country require people to be bilingual, the job goes to the person with the highest certification quite often, even if their technical skills and experience are not as good as someone without (within reason).
Particularly in client facing roles.
Generally any sort of formal certification is good for you, whether it's first aid, defensive driving or language related. At any rate it's not a bad thing to have.
It depends on the job! If you are working with English speaking co-workers and clients, and you never need any other language besides English, listing that you know Japanese gives you a cool factor but not an edge because there is no need for you to speak Japanese. However, if you work for Sony, and you frequently talk to the Japanese departments for your tasks, then knowing Japanese gives you an edge over others because they don't need to invest in translators or other extra tools as you can speak to the headquarters in their own language.
The only time knowing multiple languages really plays a huge factor though is in customer facing type of jobs. Sales/Customer service/ Tech support. You knowing say English and Spanish gives you a huge advantage over others because they can have you switch between English and Spanish queues as needed when one get's too busy.
Otherwise I would add multiple languages for the sake of having it known you do, but unless your company works with multiple languages, it won't actually give you any edge over another.
EDIT: I also want to specify that this is for US based jobs. I am sure other countries, especially in Europe where there is multiple languages being used at any given time with multiple nationalities, this would be different.
