Difficult languages... huh... I don't think Japanese is difficult. It's quite simple - the grammar, honorifics (I don't have problems with keigo), pronunciation. The writing system isn't complicated at all. It's demanding for you to learn. I know, when I write kana to people who never had anything in common with Japanese thay say "how could you learn this". It's hard for them to imagine writing anything in systems other than romaji. (It applies to other languages too).
I suppose that Korean is more difficult than Japanese. The pronunciation is hard for me, it's hard for me to write from listening, and it's hard to pronounce properly. Luckily sonsen-nim (sensei) stopped trying to teach us exact pronunciation (when for 1/3 of time, you correct pronunciation, it's hard to learn anything). There are more exceptions in Korean than in Japanese. I suppose it's harder for me to learn Korean, cause I was forced to learn this, I didn't want to learn this language.
English may be difficult because of these time aspects and it's irreagularity. Still, I wouldn't say it's an difficult language.
French looks difficult for me.
Russian is not a difficult language. Sometimes it can be misleading because of it's similarity to Polish... Other than that, I don't think so.
And Polish. Yes, it's difficult language. Because of the pronunciation, because of the grammar (gender, plural form, irregularity etc.) and writing can be misleading, like in Korean. :)