A hacked together script to change iOS simulator languages and locales. Use at your own peril.

simlang.sh

#!/usr/bin/env bash

set -e -o pipefail

checkRequirements() {
  requiredCommands=("jq" "xargs" "xcrun")

  for cmd in "${requiredCommands[@]}"; do
    if ! command -v "$cmd" &> /dev/null
    then
        echo "$cmd could not be found"
        exit 1
    fi
  done
}

simulatorUdids() {
  xcrun simctl list devices available --json | jq -r ".devices | flatten | .[].udid"
}

changeLocale () {
    local UDID="${1}"
    local LANGUAGES=("en-US" "fi-US")
    local LOCALE="en_US"
    local KEYBOARDS=("en_US@sw=QWERTY;hw=Automatic" "emoji@sw=Emoji" "fi_FI@sw=QWERTY-Swedish-Finnish;hw=Automatic")

    local PREFERENCES="$HOME/Library/Developer/CoreSimulator/Devices/$UDID/data/Library/Preferences/.GlobalPreferences.plist"
    local DEFAULTS="$HOME/Library/Developer/CoreSimulator/Devices/$UDID/data/Library/Preferences/.GlobalDefaults.plist"

    if [[ -f "$PREFERENCES" ]]; then
      echo "Found: $PREFERENCES"
      # plutil -p "$PREFERENCES"
      plutil -replace AppleLocale -string "$LOCALE" "$PREFERENCES"
      plutil -replace AppleLanguages -json "$(jq -c -n '$ARGS.positional' --args "${LANGUAGES[@]}")" "$PREFERENCES"
      plutil -replace AppleKeyboards -json "$(jq -c -n '$ARGS.positional' --args "${KEYBOARDS[@]}")" "$PREFERENCES"
    elif [[ -f "$DEFAULTS" ]]; then
      echo "Found: $DEFAULTS"
      plutil -replace AppleLocale -string "$LOCALE" "$DEFAULTS"
      plutil -replace AppleLanguages -json "$(jq -c -n '$ARGS.positional' --args "${LANGUAGES[@]}")" "$DEFAULTS"
    else
      echo "No plist for $UDID"
    fi
}

checkRequirements

for udid in $(simulatorUdids); do
  changeLocale "$udid"
done