So, when you learn another language other than your native one/s, the difficulty in learning it is basically assessed by how similar it is to your own. This is measured through mostly the vocabulary and grammar, but also through the writing systems in use (for example when a native English speaker wishes to learn Russian, they'll need to learn to read Cyrillic on top of the Russian vocabulary). For English speakers, languages like Dutch, French and German are amongst the easiest to learn. This would be followed by other Romance or Scandinavian languages, then Slavic followed lastly by everything else (Semitic, Asian, Aboriginal and so on).
So, what makes a language like Esperanto to learn? Well, it has simple and regular grammar, combined with similar vocabulary and a slightly modified latin script.
For example to say "Do you have a computer" in Esperanto would be:
Cxu vi havas komputilon?
Da li imate računar?
In the Esperanto example, there are two words that look like English words - havas, komputilon. These words stand for have and computer. While you probably would not have picked up what the question was at first glance, now knowing what the words mean makes it easy because they're so similar to the English words you use.
In the Serbian translation, there is nothing to work with. You can't go "oh yeah, that looks kinda like the English word". So to learn vocabulary, it's purely by rote memorization and force that you overcome that. On top of that, you have to worry about the grammar. The word "have" in Esperanto is the same no matter who we are talking about, but in Serbian it's different every time. Likewise, in Esperanto you can recognize if a word is a noun, adjective/adverb or verb by its ending.
Nouns end in O, adjectives in A, adverbs in E and verbs in the present tense end in "as". Knowing that this is the case every time helps deciphering the sentence. While Serbian does have a fairly regular grammar, there is no way to recognize if a word is a noun, adjective or adverb just by looking at it. Putting it into the context of a sentence of course helps distinguish this to some degree depending on how familiar you are with a language, but in Esperanto, it's obvious even if it's the only word you see. In fact, because of it's suffix and prefix system, you can guess a meaning even without seeing the word before by simply knowing its suffixes and prefixes. This isn't the same for most other languages.
Anyways, I've kinda gone into Evangelist Mode for Esperanto, but it's an awesome language; I recommend people learn it; it's eye opening.
Back on to the topic! So, as we see, the similarity of the words in another language to ours helps decrease the learning time, and combined with the grammar it can end up taking little to no time at all depending on which two languages we're talking about. Written Spanish/Italian/Portuguese are very similar, and from the accounts of some of my Esperanto friends, it can take just a few months to pick up the language due to the vocab and grammar being very close. In fact, after a little exposure to spoken form, they can pick up a lot in a short amount of time.
Anyways, I got onto this topic because for me, despite growing up hearing Serbian in the home - it's still a very foreign language to me. There is almost nothing for me to work with from my previous language studies and the vocab is pretty much alien. Sure, I do know some but I know perhaps 100-200 words (the former more likely) but I need at least 1000 to have a somewhat working vocabulary. In saying that, there have been some interesting overlaps I've noticed or found.
So, the word for "eye" is "oko", which seems to have a correlation with the English word "ocular" which is related to the eyes. Another connection is the word for pharmacy "apoteka which is similar to the English word "Apothecary", a medieval word for what can be considered the then version of a pharmacist. There's also some similarities with numbers, but I won't bore you with those.
Anyways, I'm making slow progress here which for someone who is impatient as me is somewhat killing me - especially since I progressed so quickly with Esperanto. Oh well, slow and steady eh.