Source

File: wp-includes/class-wp-theme.php

public function load_textdomain() {
		if ( isset( $this->textdomain_loaded ) ) {
			return $this->textdomain_loaded;
		}

		$textdomain = $this->get( 'TextDomain' );
		if ( ! $textdomain ) {
			$this->textdomain_loaded = false;
			return false;
		}

		if ( is_textdomain_loaded( $textdomain ) ) {
			$this->textdomain_loaded = true;
			return true;
		}

		$path       = $this->get_stylesheet_directory();
		$domainpath = $this->get( 'DomainPath' );
		if ( $domainpath ) {
			$path .= $domainpath;
		} else {
			$path .= '/languages';
		}

		$this->textdomain_loaded = load_theme_textdomain( $textdomain, $path );
		return $this->textdomain_loaded;
	}