Various assessments have been used to determine language difficulty based on the ease with which infants learn a language as their primary tongue and how challenging a language is to learn as a second language by older children or adults The American State Department has compiled approximate learning expectations for a number of languages. Of the 63 languages analysed, the five most difficult languages to reach proficiency in speaking and proficiency in reading (for native English speakers who already know other languages), requiring a minimum of 88 weeks of intensive study are Arabic, Cantonese, Mandarin, Japanese and Korean. Japanese is considered to be the most difficult of the five.
Learning a language as an adult strongly depends on the learner's native language. Therefore it is meaningless to make a universal ranking of difficulty. If you are learning Greek and already know or recall some French, you will spot many similarities in the grammar of the two languages. Native English speakers will learn German - and vice versa - much more easily than a native Japanese speaker would. In general, the closer the second language is in relation to vocabulary, sounds, sentence structure, culture, and other factors to the learner's native tongue and culture, the easier acquisition will be. When learning a foreign language you begin to learn how to learn languages in general, making it easier to understand how language and communication works.
Vocabulary improvement is undoubtedly one of the key elements in any language learning process, regardless if you're learning French, English, Spanish, Chinese, Japanese or any other language, no matter how "complicated" it is and what its rules are. Actually, obtaining an initial vocabulary base is one of the key elements you need to focus on when learning a foreign language and afterwards, increasing your vocabulary is a constant process to go through.