Time and difficulty
Related languages
It is important to take into consideration the amount of time you have as well as the amount of time required to learn a language. The time factor is in direct relation with the difficulty of the language you plan to learn. Generally speaking, it is easier for someone to learn a language that is more closely related to his/her own. If you're native language is German, you will most likely have a much easier time learning Spanish than learning Arabic even though upon first glance, German and Spanish don't look and sound similar. The fact is that, even though Spanish is an Italic language and German belongs to the Germanic family group, Spanish and German are both a part of a larger language family called Indo-European which is distinct from the family to which Arabic belongs, which is called Afro-Asiatic. On the other hand, if you must choose between languages which are equally unrelated to the language(s) you speak, a factor could be complexity.Complexity of languages
Some languages are indeed much more complex than others. In Icelandic for example, verbs are conjugated according to person. To make matters more complicated, it also makes use of "cases" like nominative, accusative, dative and genitive where a noun is written/pronounced in different ways according to its function in the sentence. In addition, nouns in Icelandic can even come in one of 3 genders (masculine, feminine and neuter) which can impact the way the adjectives associated to the nouns are written/pronounced. In contrast, Danish, which is very much related to Icelandic, doesn't have these features, at least, at a much less complex level.Some languages are completely deprived of these features. For people whose native language is without these features, it most likely will be a more time-consuming endeavor to learn Icelandic. So, it is arguable that for a native speaker of Mandarin for example, which to my knowledge doesn't have any of these features, Danish is much easier to learn than Icelandic.
There are many more features which are not shared by all languages. These can be tone, conjugation, specific phonetic sounds, sentence structure, alphabet and the list goes on. Features of the language you intend to learn should be determined in order to assess the level of difficulty it involves for you to learn it and consequently the time needed to be spent on learning it.
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