Drupal 8.x

Hello,

I try to upgrade my Drupal 8 website (version 8.9.18) to Drupal 9 (version 9.2.5) and i get the following errors :

First try

How to implement "hrflang" tag code on the website running with different languages. If there is any module in Drupal 8 that can help to resolve this. Or we can directly add the "hreflang" tag code.

We have tried to implement the code directly but the code starts to run dynamic and static both.

On the online site suddenly the callBacks are no longer invoked. I made a copy of the site on XAMPP (local) and everything works normally.
To understand what could have happened, I created a new content type with a numeric field and activated the callBack which only displays a string via the alert function, but it doesn't work.
It is as if the callback functions are invoked but not executed throughout the site. But in the local everything works normally. This all happened after I upgraded the core from version 8.9.17 to version 8.9.18

Hi!

So I wanted to use Drupal 9 but the only module that I found good for registrations is RNG which has problems in Drupal 9. 

What would be the best way to create registrations for events? I only need simple things like setting maximum number of registrants. And administrator to be able to see the people registered. And email reminder would be nice.  Am I missing something or there is only RNG for that. I would rather use drupal 9 but I already moved to 8 because of the RNG problem(feels stupid cause of the d8 losing support soon). 

We have a Drupal 8 site. I am relatively new to Drupal but learning fast! It is a composer install and the composer.json and the custom theme are in a github repository.