I just read an article about difficulties in language learning. The focus of the article is in English speaking people learning other languages, the same might not apply to others. Also the focus is on my native tongue, Finnish, which is quite difficult to learn.
http://yle.fi/uutiset/finnish_among_most_difficult_languages_for_english_speakers/6690499
According to the article, Arabic, Chinese (both Cantonese and Mandarin), Japanese and Korean are the most difficult languages for native English speakers. It will take approximately 88 weeks to learn them.
Easiest languages, with just 22 weeks to learn, were French, Dutch, Italian, Spanish, Swedish, Afrikaans and a few other similar languages.
The data comes from here: http://www.effectivelanguagelearning.com/language-guide/language-difficulty
(This is just one view, some other studies may show different results. This is mentioned in the web page.)
Here's a funny example of my language, Finnish, I'm sure there are countless other examples
The spruce is on fire = Kuusi palaa
The spruce returns = Kuusi palaa
The number six is on fire = Kuusi palaa
The number six returns = Kuusi palaa
Six of them are on fire = Kuusi palaa
Six of them return = Kuusi palaa
Your moon is on fire = Kuusi palaa
Your moon returns = Kuus
Six pieces = Kuusi palaa
Just for fun made the same sentences in Estonian, expected more similarity as they are very close languages
Kuusk põleb.
Kuusk tuleb (tagasi).
Kuus põleb.
Kuus põlevad.
Kuus tulevad (tagasi).
Kuu põleb.
Kuu tuleb (tagasi).
Kuus tükki.
