At first you will need png images for the flags . Download the flags for your layouts .
Open gimp and rescale them to 26x10 (width x height). Rename them to your layout language
like for US layout rename the image as "us.png" , for Russian rename to ru.png and etc.
Then put them in ~/.icons/flags .
After that install dconf-editor (if not installed). Open dconf-editor and go to
org/mate/desktop/peripherals/keyboard/indicator and check the "show-flags" option .
Well it is . The option is just not included in the keyboard preferences , for a unknown reason . Maybe 'couse most of the icon themes don’t have country flags in them .The function dates back to gnome 2.X as I remember and gnome-flashback has it too .
But it does not work on Ubuntu MATE 16.04 LTS out-the-box. It shows GTK_STOCK_STOP icon ().
I reported bug 1783543 to launchpad. Copying flags to ~/.icons/flagsor upgrading to MATE 1.16 from xenial-mate PPA solves the problem.
Very strange... Perhaps it's because I migrated old settings from previous installation? Is there a way I can manually delete the settings file for the language indicator?
The discussed dconf key is related to the old one. It displays text or a flag icon. @nicolaigideon is probably referring to the new one from Ayatana project. It displays icons from icon theme, but these isons are basically SVG files with text.
Thanks for letting me know that @Norbert_X ! It's a test installation, I may have enabled it somehow during experiments. Unfortunately, I have no Discord account to check the reference.