This article is written from an in-universe view. The described person or event is not real but is part of the Star Trek show.

Maltz

Maltz on TKD (left)

Maltz was the sole survivor of the Klingon Bird of Prey that attacked the Enterprise in orbit around the Genesis Planet. He was captured by Admiral Kirk and his officers and brought back to the Federation as a prisoner(1). Since that incident, Maltz has cooperated in aiding the Federation to better understand the Klingons and their language. It is through his efforts that most of our knowledge of the Klingon language is derived.

Maltz as informant

Being "in the game", Okrand always mentions new Klingon words coming from Maltz. The introduction of TKD was the first place he was mentioned as an "informant who provided all of the data upon which this dictionary is based."(2)

But not all of the Klingon knowledge is exclusively based on Maltz. The introduction of KGT explains that the contents of the book were provided by the "Galactic Society of Travel" and that a "staff of Klingon consultants and Federation scholars contributed a great deal of time" – in addition to Maltz, of course.(3)

Many of Okrand's messages and emails start explanations with "Maltz has heard" or "Maltz does not know anyone using that word", so that would imply that Maltz does give us information that originates not only on his opinion, but also on his observation of other Klingons.

When TalkNow came out, we were told that those words describing Terran things (like a toothbrush or a hair cumb) are expressions "how Klingons would describe them." ^{[citation needed]} This also implies that the information is not directly from Maltz himself, but could be from Klingons who travelled the planet. The same counts for country names and city names, which Klingons might have picked up while traveling a country.

Out of universe

During the Q&A session of qep'a' 2021 Okrand explained:

The whole thing of Maltz came about when I wrote the Klingon Dictionary, when I wrote the introduction. And someone once asked what the book is all about, beause it's a very strange book. In writing the introduction, I wrote it as if I, Marc Okrand, was the researcher about all this stuff, not a speaker of the language necessarily. And my research was based on my informant, my native speaker, who had to be Maltz, because everybody else I knew about was gone. Because they blew themselves up on the Enterprise. So I thanked him and that became the way of talking about where the language comes from that's sort of the in-universe.
Out of universe it's me, I guess, I have to say it's me. And I just use Maltz as an excuse especially if I make a mistake.

Maltz in Okrand's basement

At the qepHom 2019, Marc Okrand revealed part of how Maltz came to be in his basement. Apparently Maltz remained imprisoned and forgotten on the captured Bird of Prey throughout its refit on Vulcan, its voyage to Earth and the time travel back to the 20th century, where (and when!) he escaped. How Marc became his custodian will have to wait for another story.

During the Q&A session of qep'a' 2021 he went more into detail:

For those who were wondering, how Maltz ended up in my basement, since Maltz is from the 23^rd century and I'm not, so how can the two of us exist at the same time? If you remember Star Trek III, that movie, at the end of it, the only Klingon who was in the movie, who survived, was Maltz. Also, Maltz is the only Klingon in that movie who – while we were filming – did not speak Klingon. It was John Larroquette. He had zero lines of Klingon dialogue, there was one post production line looped in. Which is why there's two words for "thousand".
Anyway, Maltz the character wanted Kirk to kill him, Kirk said I'll do that later, and he put Maltz in the brig I guess, on the Bird of Prey. And then they went their merry way, and then eventually, ended up retrofitting... that's my own word for it, because they improved it, it grew in size somehow or other, and they took it to Earth, went back in time to go Earth to save the wales and all that.
While all this was happening, Maltz was still on the Bird of Prey, they forgot all about him. So when they went back to Earth, Maltz went back with them – and he escaped... And somehow he linked up with me, and that's why Maltz is in the basement. So it makes sense that Maltz is around.

Maltz is not...

He said he wasn't an architect himself, so he'd have to think about that. (qepHom 2009)

Maltz was familiar with the scientific study of bodily functions, or porghQeD, but he didn't consider himself terribly well versed in the field. (HolQeD v12 n4)

Maltz is not a chemist, and he said he's a bit overwhelmed by the periodic table. But I'll try to coax a few more elements out of him. (qepHom 2015)

Maltz said he wasn't familiar with Klingon virus-naming conventions, so he wasn't sure how Klingon scientists would handle SARS-CoV-2. (qepHom 2020) ➞ qoro'na javtIm

I know this will lead to questions about particular underwear (and other) styles, but Maltz is not a clothes horse. (qepHom 2022)

Trivia

Maltz spoke only one single line in Klingon on screen. This line was added in post production.
Maltz' name is pronounced matlh in Klingon: pronunciation: S (4)
Maltz is portrayed on the cover of The Klingon Dictionary, the same image that was later used on Conversational Klingon.
John Larroquette portrayed Maltz in Star Trek III: The Search for Spock.