Yasuhiro Nightow has gone on record to say that English is the language actually spoken on the Planet Gunsmoke.
The black cat that appears in every episode of Trigun is called "Kuroneko Sama" (which means "Lord Black Cat". The cat is a female and was a creation for one of Yasuhiro Nightow's first mangas (Japanese comic books) and he has stated that the cat is female. Her alternate name is "Kuroneko Kukan" or "Black Cat Space". She also makes a cameo in the video game "Wild ARMs: 2nd Ignition".
For unknown reasons, the anime contains various fictional units of measurement. The three most prominent are "iles" instead of "miles", "yarz" instead of "yards", and "double dollars ($$)" instead of "dollars ($)".
There are several references to real guns from Earth in the series, like Milly's last name is Thompson, as in the 1927 Thompson "Tommy" sub-machine gun. Milly and Meryl work for the Bernardelli Insurance Society. Vincenzo Bernardelli S.p.A. is an now-defunct Italian firearms manufacturing company, founded in 1721 by Vincenzo Bernardelli. Bernardelli company was forced into bankruptcy in 1997, one year before the Anime was produced.