I feel like that is more a product of the fact that the effort put into most other languages is greatly lower at this point relative to English, rather than English is particularly easy or other languages are particularly hard. It will all come in time.

Not effort - but data. A large amount of the dictation learning was done through users using and correcting the software. If you have 40,000,000 English users but only 80,000 Polish users you can immediately see why the software may not be up to par.

Google Voice to Text, from my experiences, is bloody amazing for Chinese. My Chinese is abysmal (think "never gets the tone right") and for short phrases/sentences I've never had Google fail on me. Maybe it fails on larger sentences but it would be difficult for me to believe.

Japanese is another language it does very well in. I can only count on one hand how many times it has made a mistake - and usually repeating myself a tad slower and with less slurring and it will pick it up just fine.

Well, my language (Danish) is pretty small and I would say Google voice recognition is notably worse than English. Sometimes it's way of pronouncing names is a bit weird but it is recognized when I said them.

So it might not work for everyone in the world but at some point soon it will properly be there for most national languages.