Language is divided into categories, which will be easy to determine their degree of difficulty to learn based on the native language you speak. (ex. If you speak French, it may be easier for you to learn a similar grouped Italic-based language such as Catalan or Italian, because much of the rules are the same.)

Language/Linguistics is based on a spectrum similar to that of the political spectrum.


Indo-European Languages (since this is the main subject of this thread)

Celtic:
Breton
Irish
Scots Gaelic
Welsh

Italic:
(Romance/Latin)
Catalan
French
Italian
Portuguese
Provencal
Romanian
Spanish

Germanic:
(Divided into two categories - West and North)
West:
Dutch
English
Flemish
Frisian
German
Yiddish

North:
Danish
Icelandic
Norwegian
Swedish

Hellenic:
Ancient Greek
Greek

Albanian

Armenian

Baltic:
Latvian
Lithuanian

Slavic:
Bulgarian
Czech
Macedonian
Polish
Russian
Serbo-Croatian
Slovak
Slovenian
Ukrainian

Indo-Iranian (split into two categories, one based on Old Persian and one on Sanskrit, which is a debated topic, so I'll just leave it to this list.)
Old Persian:
Persian

Sanskrit:
Bengali
Hindi
Punjabi
Urdu

(I apologize in advance if I blundered this list, it was off the top of my head)

If you took a look at all languages abstractly, they are just as difficult equally. They all have their rules, and even their much hated exceptions. As I had stated above, degree of difficulty has much to do with your native language. It's easier to relate to the similarities in the words, grammar rules, form, and usage. If you are a native English speaker, going completely out of your category and learning a Sanskrit language is going to give you a lot more trouble than trying to learn Dutch or German. Sanskrit has its own alphabet, very dissimilar to the English alphabet, and trying to relate the similarities would be far more difficult, even if it is an Indo-European language, it is far different, based on different things...etc.

I was once told by a very good professor that learning a language is need-based. If you were stuck in a foreign country and no one spoke your native language, you would pick up the essentials much faster than you would studying it for leisure. You may not become the most eloquent of speakers, but you will pick it up on need basis. Learning a language is just like programming, it's just a code, but instead of dealing with a device or other medium, you're dealing with humans. (Culture has everything to do with it. This is a topic that pulls at me because I have so many language theories that I think will change the world, this is one topic that I'm so very passionate about.)

You have to learn the slang and the gestures, gestures and facial expressions, how one moves, it's all part of language, and a book could never teach you that.

I could ramble on about this forever. Hope this helps.