My aged Mac won’t do anything except show the multi language screen

Something like this?:

That is a kernel panic and can be due to a number of things, both hardware and software.

First, is this the iMac 27" running macOS 10.11 shown in your equipment line? Do you know what year it is?

Some suspects:

— RAM (memory): Did you recently add RAM? Not all RAM works properly in Macs, and incompatible RAM is the #1 cause we see here of kernel panics. If you recently added RAM, remove the RAM you added and test with only the original RAM.

— 27-inch 2009 iMacs had documented hard drive issues that could cause panics.

— IF you are running macOS 10.11, that suggests the computer is quite old by computer standards. Age and heat can stiffen wiring insulation cuasing it the flake off, increasing the potential for shorting. Power supplies fail from age and heat. Componenets like RAM can suffer corrosion on their contacts.

Things to try:

• Safe Mode boot: see: Start up your Mac in safe mode - Apple Support . If you achieve a workable desktop in Safe Mode, let the computer "soak" in Safe Model for about 20 minute before attempting starting normally. That give safe mode time to do its dicks checking.
• Remove RAM modules ( Install memory in an iMac - Apple Support ) and check their contacts for corrosion or staining. If you find either, use an art gum eraser to gently clean the contacts. When you reinstall, insert and remove each module several times to freshen up the contacts in the RAM slots. NOTE: Improperly seated RAM can also cause kernel panics. It takes a good shove to fully seat RAM modules in iMacs with a RAM slot on the bottom edge of the case.

Have a look at > If your Mac doesn't start up all the way - Apple Support

Go down the list in that support article and try the solutions for different screens.

Perhaps the problem lies in the keyboard and mouse...? try a wired keyboard and mouse.