The Wiktionary entry covers a specific, internet-borne usage of moonspeak to stereotype languages English speakers treat as incomprehensible. Another source documenting that use, Know Your Meme, also notes its sometimes derogatory use toward languages with non-Latin alphabets:
Moonspeak is a somewhat derogatory internet slang term for any non-English language text. Though the term can be used with any language other than English it is typically only used with languages using a non-Latin alphabet, particularly Japanese. 1 Another usage has the term describe anything the reader finds unintelligible regardless of language, e.g. technical or legal jargon.
The article again attributes the usage to Turn A Gundam (1999-2002). So to follow up on these sources, I look at two questions related to yours: (a) is there a history of the usage before Turn A Gundam, and what does that suggest about the word's potential? (b) what does usage in the last decade look like, and does that include derogatory usage?
Earlier References to Moonspeak
It seems plausible that earlier English users might come up with Moonspeak, given the 20th century tendency to form new languages or dialects through -speak (see Oxford English Dictionary, "-speak," suffix) and moon landings, bases, and inhabitants being common motifs in science fiction and other genres.
One example is the China Moon Cookbook by Barbara Tropp (1992), where p. 20 has the header Moonspeak. The start of the page elaborates:
Moonspeak is the zany combination of Chinese, Yiddish, English, and pidgin that permeates our kitchen communication. Some of it has to do with my own fascination with language and the love of a verbal tickle. But a lot of it is the necessary oral shorthand that guides any frantic business that relies on humans and their words. Here, then, is a guide to Moonspeak in semi-random order.
The terms that are listed are mainly cooking terms, starting with moons - things wedge or crescent shaped, like cut onions or some pastries. The author approaches this term with affection for their own in-house communication style.
Moonspeak also appears in the 1997 book Rude Mechanicals by Sue Prideaux. The word seems to refer to incomprehensibility (p.8):
Everything was completely personal for the girl, or completely meaningless. Abstract, intellectual, or formal concepts might be moonspeak; indeed, so self-absorbed was she that not even gossip appealed.
The usages prior to the 2000s are sparse enough that they may be nonce usages, made up for the occasion. That said, it's notable that the usages both highlight a kind of incomprehensibility, either from mixed dialects or from being so abstract as to seem detached.
Moonspeak Now: Sometimes Derogatory
The experience Mitch describes is one I had. I couldn't recall ever hearing or seeing moonspeak used. However, there are search results that suggest varied use, sometimes derogatory and sometimes in a more neutral register.
For example, how do we understand this Meta StackOverflow comment to a Spanish-speaking programmer from Argentina, whose original post had some common L2 writer errors. One commenter (user1228, Jul 23, 2014) says
The fact that you refuse to follow the framework design guidelines (lower case property names???) irks me more than your moonspeak.
In context, the writer's errors in English are described as moonspeak. The usage suggests negativity toward infelicitous usage.
Other possibly derogatory usages can be found outside of published texts, such as on Reddit. One thread from 2019, "Damn moonspeak," refers to basic Japanese characters on a tablet as moonspeak. Another thread on r/translator referring to a bronze vessel in a form of Chinese has a top comment referring to it as "moonspeak." The commenter elaborates:
It's archaic Chinese (上古漢語) that's so old the only people who can decipher it are probably linguists who study this particular form of script. I can recognise the occasional characters (王，月) that never evolved much but hopes of having this translated fully is pretty abysmal unless you take this further.
Another commenter chimes in pointing out it could be derogatory, and the original poster apologizes and explains they use the term "to describe any rune-looking languages for cheap laughs."
Finding more flagrant derogatory usage may be harder because mainstream sites like Reddit would likely moderate blatant derogatory usage, and other sites (e.g., 4chan) are by design ephemeral. For instance, on 4chan it is common to encounter "moonspeak" to refer to Japanese. In example 1, the commenter is excluding someone who posted only in Japanese. In example 2, the context is highly negative and paired with other derogatory terms like "tranny":
That said, usage contextually distinct from human language boundaries may be more acceptable. For instance, here is the 2024 children's book Little Owl's Babysitter by Debi Gliori, where Moonspeak refers literally to the moon:
"First," said Pinny," I'll need to read it a very long and boring story in Moonspeak. Then, I'll have to make it a toasted asteroid snack with stardust sprinkles."
That book was published by Bloomsbury and the usage got past editors. Perhaps, if you were really clear on your usage and branding (focusing on the Moon), you might get away with using Moonspeak. But there is also a real risk of people seeing the usage as derogatory, particularly if they are familiar with 4chan and other venues.
