Math is far simpler than a language, and most people will have a basic understanding of how it works since it works the same way no matter who looks at it. Compare that to a language- and especially an encoded message in that language? Even if you had a perfect translation skill, an encoded message would still look like gibberish. Here's an example:
This is one of the easiest types of cyphers in english literature, so common that I figured out exactly how it worked within seconds of seeing it in its original context. (Without that context you won't be able to figure out what it says, sorry for that.)
As for negotiating, it's kind of the same problem. Even if you can translate perfectly, that doesn't help if you have no idea what's being said. How can you translate a word that relates to a concept you aren't familiar with? When the kings of two rival nations are sitting there waiting for you to translate, and you're just like; "What the fuck is mudita?"
You're both making good points, so there's no reason to become hostile.
My point with math probably wasn't very clear but I was trying to say math can't convey the 'concepts' of a regular language. For example you can't really say a sentence in math the way you can with another language.
As a rule of thumb, unless you want language barrier to be a key point of your story, you do not want multiple languages to be a thing in your story... And if there are multiple languages, you want a super convenient and widespread translation magic to exist, to make the existence of multiple languages irrelevant.
You want your characters to understand one another. It's as simple as that. No point in creating a language barrier unless it's an important part of your story.
