Which is the hardest language to learn?

While some attempts have been made, it is very difficult to determine the most difficult language. A learner's native language is very important when learning a new language as an adult, thus rendering universal rankings of difficulty meaningless. A native English speaker will learn Dutch much more easily than a Japanese native would. The closer the native language is to the target language, the easier language acquisition will be. 

The Foreign Service Institute (FSI) of the US Department of State compiled a list with 63 languages and after analyzing their difficulty for English native speakers. There are three categories of languages:

Category I: Languages closely related to English (23-24 weeks)

• - Dutch
• - French
• - Italian
• - Norwegian
• - Portuguese
• - Spanish 
• - Swedish
• ...

(30-36 weeks)

• - German
• - Indonesian
• ...

Category II: Languages with significant linguistic and/or cultural differences from English (44 weeks)

• - Czech
• - Greek
• - Hindi
• - Polish
• - Persian
• - Thai
• - Turkish
• - Vietnamese
• - ...

Category III: Languages which are quite difficult for native English speakers (88 weeks)

• - Arabic
• - Cantonese
• - Chinese (Mandarin)
• - Japanese
• - Korean

So of the 10 languages I set out to learn 6 are in Category I (German, English, French, Spanish, Italian and Portuguese), 3 are in Category III (Chinese, Japanese and Korean) and I guess Russian must be in Category II, though I didn’t find it in the FSI’s list.