The claim that it's a universal language is not meant to suggest it doesn't have to be learned. All languages have to be learned, including natural languages like English, French, etc. We pick up speech instinctively enough, but many elements like writing, spelling, the finer points of grammar and style, require explicit study.

All it means to say math is a universal language is it's not culturally or civilizationally specific. Whatever nomenclature we use, the idea that 1+1=2 is true for anyone anywhere. If we express that idea clearly enough, intelligent beings from anywhere in the universe ought to understand. And yes, reality is describable in mathematical terms. 1 moon plus 1 moon gives you 2 moons. Doesn't tell you everything, but it tells you something.