Visvanath Ratnaweera

That sounds complicated. Pl. compare your notes with the official upgrading document https://docs.moodle.org/400/en/Upgrading.

There is an excellent German community, I don't know why don't you ask there. Anyway, since we've started here, typically this happens when the platform (OS and the web server) are not configured for your locale, de_DE or whatever. Since you mention an upgrade, this is not the cause, since you must be on the same platform.

So the next question is how exactly did you upgrade? Could you post the link to the doc page? Have you by any chance took a DB dump and transferred it over a Windows machine and then restored?

What is your (server) platform, BTW?

Have you tried the simple things first: https://docs.moodle.org/311/en/Developer_tools#Purge_all_caches on both Moodle sites? And also clearing the browser cache? Use different browsers for the two sites.

How did you handle additional plugins during the upgrade? What are the plugins in use?

What is the theme? If not Boost, try changing to Boost for comparison. A new round of purge/delete caches may be necessary after the change.

Did you check the https://docs.moodle.org/dev/Moodle_3.9_release_notes#Server_requirements ? (That is for rel. 3.9, there are similar ones with 3.11 4.0, ... ) Or click on the version on http://www.syndrega.ch/blog/#php-and-dbms-compatibility-of-major-moodle-releases for your convenience.

P.S. If you identify yourself with a more hominoid identifier than "potato chips", you'll get more empathic answers, if at all.

Typically an application can not backup itself. You need to give instructions at the OS level. On Linux/Unix you automate these things by writing a shell script.