function _getRequiredLanguages() {
		$languages = get_option( 'pressbooks_global_typography', [] );
		$book_lang = $this->_getBookLanguage();

		if ( ! empty( $book_lang ) ) {
			$languages[] = $book_lang;
		}

		if ( is_array( $this->getThemeSupportedLanguages() ) ) {
			$languages = array_unique(
				array_merge( $languages, $this->getThemeSupportedLanguages() )
			);
		}

		return $languages;
	}