1884: You can have a chat with General Sheridan about, or in, Chinook Jargon
This particular idea of a “court language” doesn’t refer to speaking in the courts, with a judge.
Instead, it comes from White folks’ experience of using Chinuk Wawa to negotiate with Indigenous leaders.

Image credit: Shenandoah Valley Battlefields Association
Now, here’s General Phil Sheridan, who we already knew as a Jargon speaker, specifying the limits of his knowledge:

HE SPEAKS “INJUN.”
I showed the General another newspaper
story, which represented that he was familiar
with several Indian languages, and that when
he went among the red men he never required
the services of an interpreter.“I wish it was true,” he responded; “but it’s
only one of the many fictions that have been
printed abont me in the papers. I don’t know
why people get up these yarns,” he said, “and
it’s very provoking to be compelled to confess
that one lacks accomplishments that are uni-
versally attributed to him. The only Indian
language I know is the Chinook, a dialect that
is used by all the tribes on the Northern Pacific
slope, — a sort of court language which is used in
great councils. All the tribes understand it,
and converse together in that exclusively when
they meet in great pow-wows. I learned it
when I was serving as a Lieutenant up in Ore-
gon before the war, and have forgotten most of
the words; but those dialects are easily picked
up when one has once learned them.”
— from “Chat with Sheridan”, in the Washington (DC) National Tribune of July 17, 1884, page 2, column 5
