Which is the easiest language to learn
The question is not complete. The same as if one asks: "what is the favorite food?" May be it is the hay for the horses, the carrots for the rabbits, the bamboo for the pandas...Do you understand? Here is the same. For example
Easiest language to learn for the native English speakers is not the easiest language to learn for the native Chinese/Tatar/Arab/Tagalog speakers.
Also, it depends what else foreign language/languages you're already able to speak. For example if you can speak Tatar, then it will be very easy for you to learn the similar to Tatar - Kazakh language. Or if you speak Bulgarian, it's not difficult to learn Serbian.
So, let's say that you're a Chinese. And you're good at English and Russian. Well, in this case easy languages for you may be these close to your native Chinese (for example Bhutanese or Burmanese - they both are Sino-Tibetan languages as Chinese) or these close to English - German, Danish, Norwegian, Swedish, etc., or Russian - Ukrainian, Belarusian...
There are also other factors as alphabet (characters), for example. Some writing systems are not that easy. Here is an interesting map about it:
http://upload.wikimedia.org/wikipedia/commons/thumb/0/0a/WritingSystemsoftheWorld6.png/800px-WritingSystemsoftheWorld6.pngSummary
Description
English: Rename carakan as Balinese (Balinese alphabet) and add Carakan (Javanese alphabet)
Date 29 January 2012
Source Originally from en.wikipedia.
Author Original uploader was User:Nickshanks at en.wikipedia
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I hope it answers your question about the easiest languages to learn.