I ran into this recently when a client wanted to add over 28 different languages to their WordPress site. I got through 27 no problem but for some reason every time I tried to add a 28th it would give me a strange error about having to fill out all required fields. The error didn’t highlight any fields or tell me specifically what to add. Finally through trial and error it turned out that it was a memory issue. Once you get to a certain amount of languages you need to increase the memory for the site. The easiest way to do that is simply in the wp-config.php file. Add define(‘WP_MEMORY_LIMIT’, ‘256M’); somewhere near the top of that file and this should solve the issue.