The No-Plugin Guide for a Multilingual WordPress Site Using Multisite
https://wpmudev.com/blog/multilingual-wordpress-site-multisite/#creating-a-word-press-multisite-with-subdomain

The Ultimate Guide to WordPress Multisite
https://wpmudev.com/blog/ultimate-guide-multisite/

Open up wp-config.php and add this line above where it says /* That's all, stop editing! Happy publishing. */. If it doesn’t say that anywhere, then add the line somewhere above the first line that begins with require or include:

define ('FS_METHOD', 'direct' );
define ('WP_AUTO_UPDATE_CORE', 'minor' );
/* Multisite */
define( 'WP_ALLOW_MULTISITE', true );