"Kanji is very difficult."
Lepaslandasさんが書いたのは中国語です。No, that's Chinese, agreeing with Medusa. Generally, of course, in Japanese you expect to see kana mixed in with Kanji, or occasionally, kana only. Korean, by the way, was also written until a few decades ago with a similar mixture of kanji (K hanja) and phonetic symbols (hangeul). Now, it is written just in hangeul, except for occasional kanji to clarify meanings, for example, in the case of homophones.
There are, however, a couple of writing styles connected with Japan of long ago, in which the text is written entirely in kanji. The first is kanbun (漢文), which actually is one or more kinds of classical Chinese, not only writing that came to Japan from China along with Kanji, and many aspects of Chinese culture, but also writing produced in Japan, before a writing system evolved to write Japanese. The term kanbun, can be applied not only to the texts themselves, but also to a system that developed to help convert the Chinese text into something more comprehensible to speakers of historic Japanese, an unrelated language with an entirely different grammatical structure. Kanbun can indicate necessary changes in word order, verb and adjective inflections, and the pronunciation of some kanji. Tang poetry, for example, can be read or chanted in a kind of Japanese in this way. Though it certainly doesn't sound like modern spoken Japanese, with some study, it can be understood basically, by people who don't speak or know modern or classical Chinese. The earliest written history of Japan, the Kojiki, was written in Kanbun, according to Wikipedia.
Modern hiragana and katakana are derived in a couple of different ways from the use of certain kanji to represent the syllabic sound units of Japanese. Before the simplification or reduction of the kanji to modern kana occurred, Japanese could be written in a style called man'yōgana (万葉仮名), combining the use of kanji for meaning with some used only for their phonetic value . The name comes from its use in the Man'yōshū, a compilation of Japanese poetry dating back to the 700's.
The English translation should be "Kanji ARE very difficult" unless you are only speaking of one, in which case you would need an article or a determiner before Kanji. (Edited to add:) Yes, folks, the rules of standard English necessitating the use of a plural verb when the meaning is clearly plural apply to foreign words unmarked in form as plural in the same way they apply to the handful of English words that may similarly take the same form in the singular or plural.
There were (not was) samurai in Japan in the past.
Fish are (not is) interesting (creatures).
To refer to an individual kanji, samurai, fish, or sheep we must use a, the, this, etc.
I wish I could make the kanji larger on my cellphone. Even with my reading glasses, I can't see the more complicated ones clearly enough. They're perhaps a sixth the size of my smallest fingernail, and when they have a dozen strokes in such a small space, I just can't resolve all the details clearly. I don't have a problem with the simple kanji or the hiragana or katakana. I keep wanting to pinch the screen to make them bigger, but of course that doesn't work. Now that I've gotten far enough into the course to start seeing kanji more often, it's getting to be a real problem.
Even though the screen in the Duolingo app can't ordinarily be pinch or spread zoomed, in Android (don't know about iPhone) you can override this limitation by going into settings, accessibility, vision, and switch on the touch zoom. A triple touch is the toggle back and forth. When zoom is toggled on, you can then use the conventional two finger zoom gestures to adjust size (and to move the viewable part of the screen).
No need to be rude. The original comment was made quite some time ago--before Duo started introducing kanji. I'm actually glad that I started the Japanese course when it was brand new. I think it was easier for me to learn hiragana first and then add in katakana and kanji because I knew nothing about Japanese when I first started, but I remember that people with some pre-existing knowledge were very frustrated that it didn't use kanji in the lessons and often didn't even accept them in answers even when they were correct.