I guess that sane language would be Pascal, where := is the assignment operator (and = tests for equality, like you would expect if your brain hadn't been molded by generations of C-family languages). In Go, I guess having := is a nod to Pascal, but it declares a variable besides assigning it.
One interesting thing several modern languages did was to decide that := is actually two separate tokens where : separates an optional type and = separates an initializer, so that
my_goose : Goose = get_a_goose();
... lets us give my_goose an explicit type, while we can write:
my_goose := get_a_goose();
... and leave the compiler to infer that my_goose is a Goose because that's what get_a_goose() returns and we didn't specify.
The idea here is that we have this flexibility but we didn't burden the language with what feels like an extra feature (like C++ auto) with its own special rules. Given that := starts out as a single token this is in some sense revisionist, but so long as you're trying to learn to program, not studying the history of programming, that's fine.
It reminds me of how eggcorns can become language features. A person incorrectly analyses a word or phrase they've heard, e.g. they think the things which fall off an oak tree must be named "eggcorns" because they look a bit like eggs. They apply this analysis, and, if the results are successful and out-competes existing correct analysis, it can dominate, next thing you know† your spelling correction tool says "Did you mean eggcorn?" when you type acorn.
† In reality these transitions usually take generations
