Purpose of conditional in context
We have to assume that mathematics is a language in order to say that numbers are a part of that language:
If it is true that mathematics is a language, then an essential part of that language is numbers.
If it’s a language, it has grammar and syntax — with numbers instead of, say, nouns. If it’s not a language, we have nothing to talk about here. We would just say that an essential part of math is numbers.
“Not everyone agrees that mathematics is a language,” says Anne Marie Helmenstine, Ph.D., in ThoughtCo’s Why Mathematics Is a Language.