List of grammars and vocabularies:
English is a Germanic language, but a very big part of its vocabulary is taken from Romance languages. So the easiest to learn are Germanic and Romance languages. Most probably, the three easiest languages are Frisian, Afrikaans and Dutch.
1. Easiest of all is Frisian. It's a language spoken by just half a million people in the Netherlands and Germany. Frisian is the nearest relative of English.
2. Afrikaans – it's a language spoken in South Africa. It's a little bit easier than Dutch because of simpler grammar.
3. Dutch – language of the Netherlands and Northern Belgium.
Then it's not so certain. Either Scandinavian languages or Spanish.
Scandinavian languages (Norwegian, Danish, Swedish) have a simple grammar and their most basic words are similar. But because of enormous amount of loanwords which English took from old French and Latin, Romance languages can be easier. The good side is : Romance languages are spoken by hundreds of millions of people.
The easiest is most probably Spanish. The others are French, Italian and Portuguese. Most probably the most difficult Romance language for English learners is Romanian, but it's still easy. According to FSI, only 600 hours are needed to learn one of these languages.
Which language is next ? German. It's a Germanic language, but it has a very complicated grammar.
Most other languages are in the group „difficult to learn”. According to FSI, Malay, Indonesian and Suahili are easiest of this group – because of relatively simple grammar. The examples of languages from this group ? Russian, Polish, Turkish, Tagalog, Cebuano and many others.
Which are the most difficult languages to learn ? Arabic, Korean, Chinese (Cantonese and Mandarin), and Japanese.
Arabic has a very big vocabulary, the own alphabet and very complicated grammar.
The same concerns Korean.
Chinese and Japanese have a very big vocabulary and extremely difficult characters. One must learn thousands of characters to master their written form.