Thanks (ElevenLabs dev here), we are constantly working on improving our model, we do out own research and train it completely from scratch.
We do support Polish already and the quality is actually better IMO than English as we use a newer generation model: https://www.youtube.com/watch?v=ra8xFG3keSs
Some people think it is fake and we hired a real voice actor to read.
In any case, I'm hoping this can be expanded to other languages as it would be an amazing tool for language learning.
[0] https://blog.elevenlabs.io/the_first_ai_that_can_laugh/