Here I’m referencing all of the languages in lower tiers on the FSI charts. More specifically, many European languages tend to be in the lower tiers. In general, most languages in the Indo-European language family tend to be easier for English speakers to learn, as do languages that use the Roman alphabet. While it may be interesting to discuss, I think it would be very difficult to prove whether Japanese or Chinese is harder to learn for native English speakers (and the FSI chart reflects that by putting them on the same tier).
Because the forums are all English-based I think a lot of people forget that many people on here are not native English speakers. For those of us who are I think we often consider this implicitly, even though there are many situations (like this) where we really ought to clarify. If you speak Chinese natively you’ll obviously have a much easier time with the Japanese writing system and if you speak Turkish you’ll naturally have a much easier time with the grammar because the languages are structured virtually identically. But Japanese and English are about as different as two languages can get.
I think “European languages” was used here as a generalization of Tier 1 languages (i.e. Danish, Dutch, French, Italian, Norwegian, Portuguese, Romanian, Spanish, Swedish), which tend to be Proto-Indo-European derivatives and have a lot in common with English. Likewise “Asian languages” is likely a generalization of Category IV (i.e. Cantonese, Mandarin, Japanese, Korean, and Arabic). These categories only include languages that are taught at the FSI and thus are non-exhaustive, but of the world’s most commonly spoken languages, East-Asian languages would require going the farthest back to find a common precursor with English. I’m sure certain African and Native American languages would be even harder for an English-speaker to learn than Japanese or Chinese.
The highest (2200) and lowest (600) class-hour requirements on the FSI chart show a 3.67:1 ratio, so saying 4x is a bit of an exaggeration but isn’t wildly inaccurate, although as mentioned above it’s not scientifically rigorous.
I would be curious to know whether any other organizations publish a similar list. At the end of the day I don’t think any of this has a big impact on how I go about studying (I certainly don’t have a 2200 hour calendar that I’m marking off every day), but I do think it’s a pretty interesting topic.
Edit: The James Child paper cited in the quote I posted above sounds like it might have some interesting info about language commonalities and their difficulty, but I couldn’t find a pdf.