Welcome to the Auth0 Community!
From my understanding, you would like to add an optional metadata argument for a user with a language code, for example, that you can later fetch from the token.
Unfortunately, there’s no “easy way” to implement a dropdown apart from implementing your own signup and login screen (you will need to host it yourself). I would suggest trying a lock configuration with select custom fields that would take the role of the mentioned dropdown. After signup, the information will be stored in the user metadata profile.
var lock = new Auth0Lock(config.clientID, config.auth0Domain, {
additionalSignUpFields: [{
type: "select",
name: "language",
placeholder: "Choose your language",
options: [
{value: "en", label: "English"},
{value: "fr", label: "French"},
{value: "es", label: "Spanish"}
],
}],
...
I hope it will help you
Best regards
Dawid