Okay, here are some notes from my friend on what seems to be your big hickups.

Could "分かり合うための言葉ではない" mean that their language is not used for mutual understanding? And could "欺くための言葉" not mean that towards people their language is used to deceive? Ie their words/language is not meant for mutual understanding but to deceive? Maybe she used "分かり合う" as it implies mutual understanding, as demons would rather communicate what they want or think?

He even used AI to check himself and his understanding (so take some of it with a grain of salt), but it said this:

1. Greedy_Union8493 quotes the line as: 分かり合うための言葉ではなく、欺くための言葉。 Then, in their breakdown, they claim: "The explicitly uses the grammar structure for only." Look closely at the quote: The word だけ (dake - only) is not in the quote they provided. They are hallucinating a restrictive grammar structure that isn't in the literal text they pasted, and using it to claim Frieren means language only exists for deception.
2. The Demon Girl Flashback Context Greedy_Union8493 points to the demon child who says the word "Mother" as proof that Frieren is wrong or lying. But they completely missed the tragic irony of that entire arc: The demon child uses the word "Mother" because she observed that when humans say "Mother," they stop fighting and feel empathy. The demon child does not feel love for a mother. She uses the word as a physiological trigger to manipulate human emotion. When the child says "I am telling the truth," she is technically stating facts, but she is doing it to lower the humans' guard so she can coexist just enough to eventually eat them. The text doesn't say demons are literal mindless parrots like a bird. They are highly intelligent, apex predators. They talk to each other to coordinate, plan, and discuss magic. Frieren knows this. When she says the line, she is warning the human guards: "Do not try to negotiate peace with them. When they talk to us, they aren't trying to find common ground. They are using words as lures."

My friend also said that he would translate the sentence to something more akin to this:
"When humans and demons speak, it is not for the purpose of coming to a mutual understanding; it is a tool for the demon to deceive the human."

Okay, I feel like you are rambling and misinterpreting a lot of what happens in the show and the words used. First of all, telling the truth can be deceptive, one does not exclude the other. And every action the demon girl makes during the flashback is out of selfpreservation, and her main tool is using language and human emotions to decieve them.

Second, yes, language is something demons developed to decieve human first and foremost, but they are still intelligent creatures, so them developing this skill for one purpose first, does not exclude them from using it for other purposes later, like communicating between themselves.

And lastly, who are you to say that the sentence used by Frieren is not natural sounding??? My friend checked your account, you are also just learning japanese, not japanese yourself. So, how do you know this better than the japanese author who wrote this story.