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.