Do you get any error messages?

Ah, yes, I noted on the online version it sometimes gave the following on the home page: "0 Call to undefined function imagecreatefromjpeg()"
It often however jumped to an English language page rather than display the home page. This is what happens on my local version. Also, I use british and brazilian flags on the language switch. With the problem, only the british flag is displayed. Might the "imagecreatefromjpeg" relate to the brazilian flag?
Online, the sysadmin people have temporarily reverted to an an older version of PHP to allow our pages to work.

in this case your admin doesn't installed all dependencies, I would suggest to add gd2 lib to your php installation

Thank you.

On my system I tried:

sudo apt-get install php7.4-gd
and
sudo apt-get install libgd3

and restarted the apache2 service.

The problem of not displaying the Brazilian flag continues and nor will it display the home page. On my system I don't think I saw the error: "0 Call to undefined function imagecreatefromjpeg()"

do you have a custom template which overrides the mod_language template?

you can find this in /templates//html/mod_languages/default.php

Yes, perhaps it is a template issue. The file templates//html/mod_languages/default.php doesn't exist however. The site uses a "Joomlart" template called ja_nuevo. The directory templates/ja_nuevo/html contains various mod_* sub directories but none relating to language. I ran in the base directory:

find ./* -name mod_languages -print
./media/mod_languages
./modules/mod_languages
./templates/beez3/html/mod_languages

The template beez3 is not being used.

I also tried changing the template style on the home page that does not load. It continues to jump to the English home page http://127.0.0.1/cen.unb.br/en/ which does not show the brazilian language flag (the default language).

Please test installing on the same server a default core 3.9.20 and its default multilingual data with NO 3pd extensions.

Closing as matter is now on forum https://forum.joomla.org/viewtopic.php?p=3610594&sid=0c45ce093cb9796456cb64bf293b42e4#p3610594