How to programmatically switch language in DRUPAL8?

Question

Drupal

There is a multilingual site. How to switch language programmatically?

Answer 1

$lang_code = 'en';     
\Drupal::configFactory()->getEditable('system.site')->set('default_langcode', $lang_code)->save();

Answer 2

From the description of the question it is not very clear why you need it and what the problem is. If you described the problem in more detail, it would be easier for you to advise something.
Especially for this, there is a Language field in the user profile. And in the language detection settings, you can enable the use of the user's language (and even several detection methods and determine their order).