Validate form with preprocess_user_form
The most obvious issue with your code is that you are trying to override the entire form structure within a theme preprocess hook. This does not work. Such hooks will be able to alter the output of forms or any other templates by providing additional variables, altering existing (theming) variables, or adding theme wrappers. They will not be able to alter the processing logic of a form.

Instead, you should manipulate your entire form within hook_form_FORM_ID_alter().

The second issue is, that your implementation of hook_form_FORM_ID_alter() is missing the _form_ part between mymodule and the form ID.

Also, consider using the t() function with English texts for all labels and add your desired translations to Drupal I stead of using hard-coded strings.