Created attachment 123017 [details]
kcm_fonts adjust any language

SUMMARY
In kcm_fonts there is an option to "Adjust All Fonts...". This option is only useful for the default language/writing system (in my case, English). 
In order to set all default fonts for, let's say Arabic, we need to go to each Font Type (General, Fixed Width, Small, etc), change the "writing system" to Arabic, then change the font.

STEPS TO REPRODUCE
1.Open System Settings 
2. Fonts > Fonts
3. General > edit (pencil) 
4. Writing System > Arabic 

OBSERVED RESULT
To set all default fonts for any given writing system/language each font type (General, Fixed width, etc) has to be modified individually.

EXPECTED RESULT
In the current configuration it is hard to see at-a-glance what the default fonts are for any given writing system save for the default language/writing system.

I am suggesting next to the "Adjust All Fonts..." button, there also be a Language drop-down list. That will allow users to use "Adjust All Fonts..." for all languages AND it would also allow for quick looks at what fonts are set for what languages. 

Currently this can be achieved by setting fallback fonts in the fonts.conf file.
https://wiki.archlinux.org/index.php/Font_configuration/Examples#Japanese

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Arch Linux
(available in About System)
KDE Plasma Version: 5.16.5
KDE Frameworks Version: 5.62.0
Qt Version: 5.13.1

ADDITIONAL INFORMATION
Attached is what I have in mind for the drop down menu.