Having lived in several countries I find that local salaries and costs move in line, so I find the financial case is largely irrelevant
Big mistake. That only works for comparing first-world countries like the ones you mentioned, and breaks down when you compare poorer countries like Poland. You'll find that the cost of living in Poland relative to wages is much higher than in Western Europe.
The thing that is going to hurt the most is how little in terms of absolute dollars you will be able to put away at the end of the month. Your absolute savings potential will be a small fraction of what it would be in the West. If there is going to be a deal breaker, it is precisely that.
the language is very challenging for native English speakers
Unless you are planning to live there for ten or more years, at least, forget about learning the language. It's not a plug and play language like English where students can use what they learn very quickly. The grammar is perverse, so saying even the simplest things requires abundant mental gymnastics.You would probably leave before you could string simple sentences together. And the language is not very portable. Same goes for Czech or Hungarian.