<.There are countless children whose native language is not given as AP course.>
These kids take others tests. I have Russian friends. They take college test to prove fluency in Russian. UCLA is known to give such test, but I am sure that other colleges do the same. BTW, language proficiency tests can be arranged distantly. College sends exam package to a third party (community college, for example) that would organize proctored exam and send material back to college for grading. Is costs, probably, around $20. Cheaper than AP test.
My D’s second (first?) language is Spanish. Her third language is a Native American language of my mother. I can’t find any place in USA that could test her for proficiency in this language. Honestly, I am really puzzled how to put this language on her college application. However, such problem is rare. Most languages can be tested in colleges.