It skips letters, does not read the end, does not read at all in Firefox - Polish language, on Arch Linux

[look997]

I installed TextToSpeech Pied on my Arch Linux, and selected the Polish voice. The voice is available via the `spd-say "Hello world!" command, but it skips some letters and won't read longer texts in full. In Firefox Reading Mode is available, but it doesn't even start reading at all. How to fix it?