Testing the Map Forecast API with trial key, I am unable to change the language, specifically to Japanese. Using the following options.

const options = {

    // Optional: Initial state of the map
    lat: 50.4,
    lon: 14.3,
    zoom: 5,
    lang: '日本語',
};

Have also tried lang: 'jp' and lang: 'Japanese' with no changes to map or menu language. Am I using the correct lang setting for Japanese, are language other than English not possible with trial key?