Created a seprate plugin with name profile and here is my boot code.

public function boot()
{
    UserModel::extend(function() {
        $model->hasOne['profile'] = ['Acme\Profile\Models\Profile'];
    });

    UsersController::extendFormFields(function($form, $model, $context) {

        // if (!$model->exists)
        //     return;

        if (!$model instanceof UserModel) {
            return;
        }

        // To ensure if method exist in model
        ProfileModel::getFromUser($model);

        $form->addTabFields([
            'profile[language]' => [
                'label' => 'Language',
                'tab' => 'profile',
                'type'=> 'text'
            ],
            // 'profile[currency_preference]' => [
            //     'label' => 'Currency',
            //     'tab' => 'Preferences',
            //     'type' => 'text',

                // 'type' => 'dropdown',
                // 'dependsOn' => 'country'
            // ],
            // 'profile[homepage_preference]' => [
            //     'label' => 'Homepage',
            //     'tab' => 'Preferences',
            //     'type' => 'text',

                // 'type' => 'dropdown',
                // 'dependsOn' => 'page'
            // ],
        ]);
    });
}

Created a relationship in a profile plugin

 public $belongsTo = [
    'user' => 'RainLab\User\Models\User'
];

Whenever I am saving it it says following error.

Unexpected type of array when attempting to save attribute "profile", try adding it to the $jsonable property." on line 519 of 

Even I added the following code in model

  protected $jsonable = [
    'profile'
];