yep, now I will give you some links and you can compare all differences.
Now I will answer from the start and I will repeat you words
The first and easier would be the last one you mention, that just like German and German (Austria) some countries do need their specific Serbian language version, is that right? A pull request with only these changes would be non-controversial to merge I guess.
Yes, as you said , it would be best options to do with Serbian language..
So it could be made like
Serbian (Cyrilic) or (Српски језик) <---- translation is "Serbian language"
Regarding the rs language that you made up, I'm rather skeptical of merging that. Is there any formal process being made to get in contact with ISO and get that code in the official list? If the idea was only to be able to have on the same listing both the name in Cyrillic as well as Latin, for other languages with digraphia (I had to look it up on wikipedia :D) like Urdu, we only use one writing system.
Regarding codes take a look first this source https://en.wikipedia.org/wiki/Serbian_language
and you will find ISO codes
And after that this one specially second part of page, below the table https://en.wikipedia.org/wiki/Country_codes_of_Serbia
So I can suggest only that Plone could have Serbian in two options
Serbian Cyrilic (Српски језик)
About code I can't give you any suggestion cause you can see on official ISO site how is going for Internet, cause even domestic domain name for Serbia is .RS
The whole data structure would have to be completely changed to support different locale names for a single language.
I don't think so you have to change all structure of site but I am not sure of this part of Plone structure which needs to be change, but I can show you on this link how it is look like Serbian language officialy for Wordpress and I know the guys who are working on it and there is code ( sr_RS) because this makes differences from Suriname.
Also I have registered domain name for Plone at Plone.RS and it is online already. Our domain is .RS as it is for Germany .DE
Is that something really important for Serbians, or like is stated in the Wikipedia article, all serbians are able to read equally fluently in both writing systems?
For Serbian community is only important to have two options as you mentioned before in case of Austria (Germany language)
So if there is any chance to make something like Serbian Latin and Serbian Cyrilic it would be perfect.
I have in plans to make meetup and other events of Plone for Serbian community so I think the language is essential things if we want to develope something good.
So it is all up to you... ...Plone
I can only contribute and help