I wanted to be a little more exclusive, but more specific sources are hard to come by. From what I've found, the easiest Slavic languages are Bulgarian or Macedonian, while the hardest is probably Polish. The Uralic languages, Turkish, Maltese and Basque are not related to English, increasing their difficulty. Some or all of the Celtic languages, as well as Uralic languages Karelian and Sami are likely difficult enough to be on this quiz, though they do not have enough speakers nor data.
English,Italian,German:Pony
Spanish,Serbian,Bulgarian,Ukrainian,Russian:Poni
French:Poney
Slovak,Czech:Poník
Europe: Wait,what is that? No, don't tell me it's HIM again...
Polish: KUCYK