How to set default language

You can set the default language for the conference through the configOverwrite property. Please see below a snippet on how to set the UI language to German:

const domain = '8x8.vc';
const options = {
    ...
    configOverwrite: { defaultLanguage: 'de' },
    ...
};

const api = new JitsiMeetExternalAPI(domain, options);

Note:

On joining a meeting, if defaultLanguage is not passed to the iFrame, the default browser language will be used and stored in the local storage on *8x8.vc* domain with a key named language. If on a subsequent join we pass the defaultLanguage with a different value than the browser's, it will not be considered because the value stored in the local storage takes precedence over the passed on value.