I wonder how does it go regarding second, or third.. I guess in English, which I studied as "the second language", it counts as a second language in addition to first language. But now many kids are bilingual(I'll put my best effort in to assurance of this destiny for my children), so if they study any third language wouldn't it be called second?
And overall, can't you say "I'm studying another second language", while referring to your third language? I think it's understandable. Not sure though.
我学中文 (I study Chinese.)
as my fourth language, although arguably I don't know any of the previous three. But usually people understand me.
All my Primary and High School were given in a bilingual model so maybe that influenced.
I'm highly surprised nobody mentions Chinese, which is essentially my only choice of study. English was kind of a must in Israel(thank God it was), I speak Russian because I was born in Saint-Petersburg. I'm 27, and about half of my life I spent in Israel.
And Chinese. Well, I know English good enough to read in it. And Chinese feels like second in influence in the world, if not yet then soon to be. I bought a router on Alieexpress, and it doesn't seem to have an English interface, and changing its firmware is hell of a story.. China is also the biggest economy for a while now.