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.
Alright so the common trend I'm seeing here is that Baltic and Slavic languages are difficult to learn. Turkic/Uralic/Semitic/(Basque) languages aren't Indo-European so obviously they're going to be harder. Albanian and Greek are unique enough from everything else to be their own language family, so it makes sense that they're difficult to learn.
The only thing that breaks this trend is Icelandic. It's the only Germanic language on this list, which is odd. I would have assumed that English and Icelandic would be a bit closer. I thought the influence of Old Norse from Viking invasions would have connected the languages a bit more, especially since modern Icelandic and Old Norse are so closely related. I guess Old Norse didn't impact English as much as I thought.
Icelandic is also a more conservative and insular language compared to the other Germanic languages, and so it preserves features which are more foreign compared with English.
The only thing that breaks this trend is Icelandic. It's the only Germanic language on this list, which is odd. I would have assumed that English and Icelandic would be a bit closer. I thought the influence of Old Norse from Viking invasions would have connected the languages a bit more, especially since modern Icelandic and Old Norse are so closely related. I guess Old Norse didn't impact English as much as I thought.