To get the currently selected language on a Drupal 8 site you need to run the following.
$language = \Drupal::languageManager()->getCurrentLanguage()->getId();
This will return the 2 character ISO language code for the language. So for English this would be "en".
To get the language name instead of the language code use this.
$language = \Drupal::languageManager()->getCurrentLanguage()->getName();Comments
It,s really helpful. Thank you so much.
Short, cool and informative. Thank you!
Small remark,
To get the currently selected language for content :
$language = \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->getId();