Your reasoning about Poland would also fit Germany, yet it's a very diverse country in the cities now... Also has language with very long words with a lot of consonants ("Angstschweiß" "Weihnachtsschmuck" ...) and they didn't really get successful colonies going (Namibia perhaps the most). They also carry quite the "reputation". I think for most European countries current diversity has more to do with inviting Gastarbeiter (Italian, Turkish, Moroccan...) and/or Soviet style topdown relocation programs of millions of people across the country (Siberia ...), and somehow Poland had few of both those scenarios? Anyhow I don't think difficulty of pronouncing polish language is the cause of low diversity.