doctype html lost lang=”de-DE” Tag

Dear Support, after having installed version version 4.5.3 of Adventurous Pro theme I lost the lang="" Tag in the !DOCTYPE html at https://www.stiftung-meeresschutz.org/ This causes problems with some plugins. I use a child theme and in the header.PHP I found code with: ?php language_attributes(); Could this be the cause? Best regards, Ulrich