Wow thanks for this as it's not at all mentioned in the help text for these (Configure editor settings to be overridden for a language.). Also what a strange choice for multiple languages since there can clearly be conflicts this way:

    "[javascript]": {
        "editor.defaultFormatter": "esbenp.prettier-vscode"
    },
    "[typescript]": {
        "editor.defaultFormatter": "esbenp.prettier-vscode"
    },
    "[javascript][typescript]": {
        "editor.defaultFormatter": "something.else"
    }

If they're going to allow conflicts anyways, just invert the structure so you at least get to use normal arrays:

    "editor.defaultFormatter": {
        "esbenp.prettier-vscode": ["typescript", "javascript"]
    }

Though, I guess the logic is that the other format allows you to override ANY setting based on language.