What is the most difficult part of learning English for you? (Grammar, pronunciation, vocabulary, listening...?)
What is the most difficult thing in English to you?
What are the problems that are causing you the most difficulty?
I can´t see how one would have "more difficulty" or "the most difficulty" in terms of comparing adjectives - "more difficulties" would be used in terms of "greater or additional amount" (or degree). Ex:
A native speaker of Chinese, for example, may face many more difficulties than a native speaker of German, because German is more closely related to English than Chinese. This may be true for anyone of any mother tongue (also called first language, normally abbreviated L1) setting out to learn any other language (called a target language, second language or L2).
I don´t see a way to
If you write "the most difficulty" and submit it to a spell checker it will flag it as a candidate to correction and will suggest "the most difficult".