wekan/client/lib/i18n.js

// We save the user language preference in the user profile, and use that to set
// the language reactively. If the user is not connected we use the language
// information provided by the browser, and default to english.

Meteor.startup(() => {
  Tracker.autorun(() => {
    const currentUser = Meteor.user();
    let language;
    if (currentUser && currentUser.profile && currentUser.profile.language) {
      language = currentUser.profile.language;
    } else {
      language = navigator.language || navigator.userLanguage;
    }

    if (language) {
      TAPi18n.setLanguage(language);
      T9n.setLanguage(language);
    }
  });
});