Does self-teaching a language impress admissions?

Hey there once again.
I’m teaching myself latin and french because these languages are the ones that most interest me.
During my high school years, I went to a language school since I’m from a bilingual family.
I was wondering if this background would help my application out?
Also, how do I prove that I am learning these languages? Do I need to take a test of some sort ?

No idea how it’s viewed, but you could take the corresponding SAT subject test to demonstrate proficiency, then note somewhere on your application that it is self-taught.

SAT subject test
AP Language &/or AP Lit test
CLEP Exam https://clep.collegeboard.org/exam
National Latin Exam http://www.nle.org/
National French Exam http://frenchcontest.org/fcntst/
French level exams http://www.afusa.org/