How about this -- Is speech language universal?
Yes *tongue-in-twist*. Well, if you're talking about the modality that hearing people use speech more than signing. If you're referring to language, the answer is obviously no.
Of course, when hearing people ask the question "Is sign language universal?", they mean the language itself, not modality. For the sake of clarity, let's refer "signing" and "speech" to as modalities and "signed language" and "spoken language" as languages.
By modality, there are three forms: written, spoken (vocal-auditory speaking), and signed (visual-spatial speaking).
It's not unusual that many hearing people assume that either sign(ed) language is universal or wonder whether it is. The term "sign language" is somehow vaguely referred to both language and modality that become tangled.
"I know little sign language." "I learned some sign language." "Growing up, I've been learning English and French. Now I'm learning sign language."
"Which language?" I (as the ASL instructor) asked to help deconstruct the student's mindset. Previously I taught ASL students, "I'm learning ASL."
The student replied, "sign language," I nodded, "which language?" The student, looking puzzled, repeated it. Irrestistibly, I repeated, "which language?" She/he, looking somehow more confused or sometimes frustrated, "sign language?". Generously, I repeated, "which sign language?" If a student doesn't get it, usually their classmate helps reply, "ASL." Often, the student's light bulbs in their eyes flashed.
So, yes, just like spoken languages, there are differnet signed languages around the world. It's not universal.
Most of the time, if not virtually all the time, hearing people accept this fact and move on. Fantastic.
But, worse, a few hearing people would find this fact a sore disappointment. "Would it be nice if sign language is universal? So that everyone around the world can communicate with each other. Wouldn't it be fabulous?"
Again, which (signed) language? Wouldn't it be nice if English is universal? No.
Language is culture interwined with history, traditions, beliefs, values, heritages, politics, etc. Language is identity.
Bilingualism is the answer. But, monolingualism of one language around the world? Not happening.
Bilingualism, translation technology, interpreter, Esperanto, and ISL can help foreigners communicate with one another.
We, Deaf ASL-speaking people, value signed languages of their own countries. Languages, cultures, and universal respect are a beautiful and exciting experience.
Takeaway: Make a habit of specifying a (signed) language. Use the term (e.g. Auslan, French Sign Language, German Sign Language, etc) instead of "sign language".