There are two language packages for Serbian language you can find as far as I know. Both of the them doesn't work. First one is the "official" magento/crowdin project which I found useless, as it has only small portions of string translated. Second one is from mageplaza, and that one is a bit messy. I don't like the way some of the strings are translated, as well as there are translations for admin section which i don't need anyway.
If you run: bin/magento info:language:list, you will get following output:
As you could notice, the language codes for Serbian language is sr_Latn_RS and sr_Cyrl_RS not sr_CS which I think should be sr_RS. Anyway, what I have found the best so far, is that I translate the strings manually in 'sr_Latn_RS.csv' file created in `frontend/vendor/theme/i18' directory to create a language dictionary. I use console tool to collect phrases from modules I have a need to translate and than I used regex to append the appropriate suffix for module declaration at the end of the string. That way I have a full control over the translations, but it's very time consuming task. The language package could be created from that dictionary later on and published as composer package which could be installed in vendor directory afterwords.
What would be the best, in my opinion, is to have separate translations for front and back-end areas.
If you already find appropriate solution in that regard, please let me know.
Hope this help a bit.
