One of slavic languages. Official language of late USSR, now official language of Russian Federation. 

Russian has three genders and six cases -- nominative, genitive, dative, accusative, instrumental, and prepositional. Anybody who has exposure to the grammar of any of the Romance languages won't have any problems except for the instrumental, which doesn't "map" comfortably onto any more familiar construct that I know of. 

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http://www.worldlanguage.com/Languages/Russian.htm outlines major difficulties of RussianLanguage. 

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Written in CyrillicAlphabet, which is actually quite easy to learn. Once you learn it, you discover you can recognize a lot of written words. Thus, strong recommendation: If you are going to visit any country using cyrillic letters, do yourself the favour and spend the 1-2 hours necessary to learn the alphabet. Search the net, there are free tutorials somewhere. It makes all the difference between being relatively helpless on your own and being able to read street names and recognize hotels, restaurants, pharmacies etc. from signs. 

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I studied Russian at the Defense Language Institute in 1971-72, in an intensive course, and found that I could write faster, more legibly, and more beautifully in Cyrillic than I could in Latinic. There's something about the script that feels more natural to my hand and once you start using it, you'll find it very seductive. Sadly, my skills have really eroded over time, but I still find myself doodling in Cyrillic just because it is so much more elegant than my normally arthritic chicken scratches. -DonOlson 

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Coming across this page by chance, it makes things look a sight too easy: the Russians themselves describe their grammar as consisting of exceptions (think English spelling). For example:

*Evidence of at least one other vestigial case - the partitive genitive which distinguishes between a loaf of bread and the taste of bread, a lot of people and the will of the people. 
**''Hm... Yeah. Bukhanka khlebu/khleba and vkus khleba. It looks like dative for me. Anyway, using this decilnation is not compulsory.''

*Heroically confusing numbers. One is a sort of pronoun, two is a pronoun merged with vestigial dual number (ie between singular and plural) as in ancient Greek. Three and four are sort of neuterish nouns followed by the genitive singular ''(as well as two)'', five onwards similar but more or less feminine and taking the genitive plural. The whole lot starts again at twenty-one, 101, 1001 and so on. The gender of numbers matters because they all decline fully, in ways that are similar to but always different from their nearest pronoun and noun analogues. The final gotcha is that a number declines in the singular, while the objects it governs declines in the plural. 
**''Declinations are hell, truly. People from ex-USSR coming here can't decline and conjugate properly, even though they studied Russian at school.''

*The lack of a present tense of the verb to be; the past tense that is more or less an adjective agreeing in not person but gender with the subject; the minimal importance of tense distinctions when compared with aspect (as in English I did it, have done it, was doing it). 
**''If there's no verb, than it is that missing "to be". Old Slavonic:"Az yesm' Tsar'." Russian: "Ya Tsar." (add dash if you wish). I don't agree with you on the past tense. It doesn't work like an adjective in any way but agreeing in pgender. Englishmen find it hard to narrate in Russian using only one past tense, but on the other hand many Russians learning English find it difficult to  choose the right tense. We had four past tenses in Old Slavonic, but they disappeared.''
**Lack? There is "Yest'", equivalent of "is", but using it in most places as in English will sound unnatural. -- nuclight

Compared with most great languages Russian is wonderfully unregimented and thus a powerful example of how human language combines the logical and the arbitrary (and recursively at that). 
''No strict order of the words in the sentence, but changing the order may shift the meaning. If something doesn't conjugate, it declines. Read as it's written, but beware of certain letter combinations that are in every adjective and pronoun. Declination is one big exception, animate an inanimate nouns decline differently and so on(once I tried to program a decliner... I failed miserably.)  Good luck. No, it's not that hard as it may look. But it's harder than you expect.''

Given that four-year-olds cope with complexities such as the above without even noticing, it may be that AI is still further away than we think. 

David Wright (comments in italic — Alexei Marine)