What version of Go are you using (go version)?

go version go1.12beta2 linux/amd64
golang.org/x/text master (6c92c7d)

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/dave/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/dave/Source/Go"
GOPROXY=""
GORACE=""
GOROOT="/usr/lib/go-1.12"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go-1.12/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build800313732=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package main

import (
    "fmt"
    "golang.org/x/text/language"
)

func main() {
    region, _ := language.AmericanEnglish.Region()
    fmt.Println(region.ISO3()) // expect "USA", got "US"

    region, _ = language.LatinAmericanSpanish.Region()
    fmt.Println(region.ISO3()) // expect "ZZZ" (maybe?), got "419"
}

What did you expect to see?

The documentation states:

ISO3 returns the 3-letter ISO code of r. Note that not all regions have a 3-letter ISO code. In such cases this method returns "ZZZ".

What did you see instead?

ISO3166-1 alpha-2 in the first example, UN M.49 in the second.

Also worth noting that the docs should probably clarify ISO 639-3 vs. ISO 3166-1; "ISO" is ambiguously used for both.