{
  module: "MMM-WunderGround",
  position: "top_left",
  config: {
    lang: "sw"
  }
}

the full list can be found here https://www.wunderground.com/weather/api/d/docs?d=language-support

also the config option lang has no effect at all as it is not used anywhere in the code, maybe it is from an old version. The language which is used for moment js is the global config language.

But it still shows in English and not in Swedish?

I just checked my translations folder in the MMM-WunderGround folder. I only have de, en, fr, nl and pl. This may be where your problem is.

But it still shows in English and not in Swedish?

Yes, that’s odd. You could try removing lang: ‘sv’, from the WunderGround portion of your config.js.

Also, are you sure you’re using the most recent version of WunderGround?
In your terminal, go to ~/MagicMirror/modules/MMM-WunderGround $
and type git pull, then restart your mirror

But it still shows in English and not in Swedish?

The language of the days. Goes in your config.js for WunderGround

lang: ‘en’, Change this to your language, or change language: 'en', at the top of your config.js, unless you want everything else in English.

.MMM-WunderGround table.small .wi {
    font-size: 10px;
}

.MMM-WunderGround table.small .forecastWeatherIcon {
    height: 10px;
    width: 10px;
}