Introduce language methods on user model

[ElwynVdb]

Overview

This pull request builds upon the functionalities introduced in a previous PR submitted to the web-SDK, which added language functions. It aims to provide access to these newly introduced functions directly from the front-end SDK.

Benefits

• Set the language in the front-end when you don't want to work based of the browser language.

Related pull requests

• SDK PR: [https://github.com/OneSignal/OneSignal-Website-SDK/pull/1171]