Books on leadership will often assert that anywhere from 2/3 to 80% of communication is nonverbal - that is, expressed in tone of voice, posture, eye contact, and gestures rather than the specific vocabulary, conjugations, or grammatical structures used. Are questions about learning or practicing such nonverbal communication on-topic?
To be clear, I'm not talking about signed or manual languages such as American Sign Language (ASL) that are not spoken but have specific grammar and vocabulary, but about so-called "soft skills" in communication. For example, see this article from The Balance Careers.
Obviously, we already have a close reason for "grammatical rules or specific characteristics of a language", so questions like "Please interpret the posture of the person in this video" or "For how many seconds should I make eye contact in order to convey sincerity?" are likely already inherently off-topic, but I could see someone asking questions like "How do I find a partner to practice eye contact with?", "How do I assess my level of ability in reading someone's attitude to the topic of discussion from their posture?", or "Is there a certification in using body language effectively?".