@KamilRT
Welcome to the Serif Affinity forums
Although the software translation looks relatively simple, there are tens of thousands of words in the software, and tens of thousands, perhaps hundreds of thousands in the help files and websites.
We do appreciate the offer, but we have no plans to add other languages at the moment, as it's a big undertaking and requires cross team planning.
If we ever do translate to Czech we would want to pay professionals, but you could be involved with testing and checking.
