Post installation

Im trying to change the language switcher on my website to not be text but instead look like flags. I have no idea how to do it. I have languages set up using the language module.

link: http://consonare.evangnet.cz/cs

Can anyone help me find the module that enables adding classes to a wide variety of entities in the Drupal site?

When I use most distributions (ie Varbase) I have the ability to add classes to things like blocks on the main page layout.

Trying to build up a new site based on core installation I am having trouble locating the module that provides that feature.

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Another feature I cannot seem to find is stacked layouts when using the layout builder.  They must be provided by some module but I cannot find them.

I want to create a variable with the value of (in pseudo code) CONCATENATE(month - MM, day - DD) from a date variable in one of my content types so I can sort my View by this new variable, i.e., list the records in month/day order regardless of the year.

I know I could have I have the user enter the information manually, but that seems like an inelegant solution.

Is that something I can have the system do when I add a new record of this content type so it's stored with that record as a variable? 

Do I do it somewhere in the View definition?

I want to clone the Content region to have two or more Content regions; each clone will have the same default CSS of the Content region and will contain different blocks than those in the original Content region.

How to clone a region easy and fast in Drupal 10? By easy and fast I guess I mean no Twig coding at all.

My site is heavily dependent on book navigation, which I understand is to be moved to a contributed module rather than core.  I have some ideas on how to proceed but I would like to hear any suggestions as to best practices as it appears to require editing all of my nodes that are part of a book.