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Are there any studies which address which factors—if there are any—have more influence on the learner to consider a new language as difficult to learn?
Factors could be:
There are lots and lots of factors. I am going to list some of the major ones here:
The language you start with: For example, Mandarin Chinese is ranked one of the hardest languages to learn as an English speaker. Mandarin Chinese could also be very easy for someone else with another language. Even English might be hard to some.
Psychological Barriers: A lot of times, you get fear that the language is super tough and you will utterly fail every time you try. This type of attitude makes languages a lot harder for these types of people.
Major differences: Compare Chinese to English, for example. The Chinese alphabet has thousands and thousands of characters plus different tones that are used. These are some of the major differences between these two languages, as English has only 26 characters in its alphabet.
Need, Culture, Motivation: Some languages are hard to learn because you don't really need it, your culture gets in the way, or you are just not wanting to learn that language.
More can be found here: http://www.lingholic.com/hardest-languages-learn/
I would like to add on to @PythonMaster's answer with this point.
In addition to what has been listed by PythonMaster and SMS von der Tann, I would add the availability and quality of resources for studying the language. This includes the availability of courses (classes, online courses) and reference works (monolingual, bilingual and learner's dictionaries, grammars), as well as the willingness of speakers to help others in learning the language.
To give an example from personal experience: I was interested in learning a language that is intermediate between English and German (to help me with German). I gave each of my choices (Afrikaans, West Frisian and Dutch) a try by studying a little bit of each. Now, Afrikaans and Frisian allegedly have a simpler grammar than Dutch, which would make them easier to learn. On the other hand, there is an unbelievable amount of free and high-quality material for learning Dutch online, and there are many, many Dutch speakers almost desperate to help me learn their language. For this reason, I was able to improve my Dutch much faster.
