That public repository contains translations provided by the community. You can use them as a base and also contribute your own - both new translations, and updates to older ones that contain new keys. We welcome contributions and pull requests.
At the moment, it seems that, from the community, no one has added the Slovak language yet. You can use this opportunity and add it.
We've let our customers create the TelerikMessages as the different scenarios need different languages and keys. Every customer best knows which localization keys he needs.
I hope this is helpful for you to move forward. Please let me know if you face any difficulties on the go.