Account settings can't be saved if content_translation module enabled

Problem/Motivation

STR:
- install D8 with Standard profile
- enable content_translation module
- visit admin/config/people/accounts
- submit the form
Expected result: settings are saved
Actual result: form just reloaded, settings aren't saved

What happens:
- content_translation_language_configuration_element_process() adds additional (actually the only one) submit handler to the submit button (code)
- FormBuilder::doBuildForm() checks if there are submit handlers on the submit button, and overrides form submit handlers with the given ones (code)
- FormSubmitter::executeSubmitHandlers() ignores $form['#submit'] handlers if there are handlers in $form_state->getSubmitHandlers() (code)

Proposed resolution

Option 1: Explicitly set submit handler on the submit button in ConfigFormBase::buildForm(). For example, EntityConfirmFormBase::actions() does it in this way.
Option 2: Change behavior of the content_translation module?

Remaining tasks

+ write patch
- decide if tests are required