This depends very much on what your needs are, and also on whether you are a Slav yourself. If you want to write a text in Interslavic, there is no need to really learn it. All you need is the grammar and the dictionary. Don't be afraid that the result will be bad, because there is no such thing as bad Interslavic: remember that Interslavic is not a hermetic language, but rather a set of recommendations you can follow at will. If you try to follow them, the result will in all likeliness be better than if you don't.
To be proficient in Interslavic, you will of course have to learn it. If you are Slavic or at least have good knowledge of one or more Slavic languages, this is is pretty a much a matter of gradually learning how to modify your own language in order to make it more accessible for other Slavs. Our experience is that most people quickly and easily find out how to do this by simply reading and listening. Of course, reading the grammar and/or the tutorial can help, too!
If you are new to the Slavic languages, you obviously cannot take any existing Slavic language as a starting point. A beginner's course is in preparation, but in the meantime, just read the grammar and the afore-mentioned tutorial carefully. You can also start with learning Slovianto, a highly simplified form of Interslavic that allows you to start communicating on a very basic level, while gradually enlarging your knowledge and skills.
When it comes to speaking, one has always to remember that communication is not just a matter of language. The non-verbal part is equally important. When you try to use Interslavic in a conversation, always make sure that the person you are talking to actually understands you. Speak slowly, keep eye-contact, articulate well, and always be a good listener.
The short anser is: no. There is a lot of Pan-Slavic vocabulary, but if all Slavic languages used the same words, there would be no need for a separate Interslavic language. All we have done is selecting words that are understandable to the largest number of Slavic nations. Inevitably, some words are more geared towards East or West, North or South. If a writer consistently uses words that are best understood by one particular half of the Slavic population, this will automatically lead to better results in this particular half and worse results in the other.
Understanding Interslavic is largely a matter of understanding the general meaning of a sentence even without understanding some individual words. This requires a certain level of intelligence and experience. People who have problems understanding their own language when written or pronounced slightly differently, cannot reasonably be expected to understand the same text in a language like Interslavic. Any Interslavic text will inevitably contain words that a Slavic speaker cannot link to his own language, and the ability to fill in these lexical gaps varies from person to person. Every now and then, discussions take place on the Internet about about some Interslavic text fragment, and the answers are often conflicting: one person claims to understand every single word, another person of the same nationality understands only 40%.
Also, listening to a language one does not actively know requires a level of concentration that can be achieved only if the listener is willing to cooperate. A person who has something to gain from the conversation will try harder than a person who is tired, annoyed and uninterested. The speaker should always help the listener as much as possible by speaking slowy and clearly, constantly being aware of the fact that the listener needs some time for processing.
All we can say with certainty is that a vast majority of those who have commented on Interslavic or participated in our research projects can understand texts in written or spoken Interslavic reasonably well. There are no huge differences in intelligibility between speakers of different Slavic languages, except that Czechs and Slovaks score a bit higher and South Slavs a bit lower than average. What we do know, however, is that there is correlation between people's capability to understand Interslavic and their level of education.
One recurring argument in discussions about Interslavic and artificial languages in general is this: why would anyone learn it, since we already have English as a world language?
That is undeniably true, and if you think you can get by with English in the Slavic world, then by all means do. We surely will not try to stop you! However, please consider that in Central and Eastern Europe things are rarely that simple. Many people in that part of the world are completely monolingual, and even those who claim to know English often won't be able to produce more than just a few broken sentences in it. That is especially true at the countryside, but in cities, shops and even hotels this situation is far from exceptional either. In general, the further East you travel, the worse it gets. In the Russian Federation, for example, only 5.5 % of the population can speak English (according to the 2010 census). Although this situation is likely to improve in the future, it will take many decades before English will be really helpful to those visiting the region.
Another thing is that many Slavs find it sort of shameful to use English in communication with other Slavs.
English is a highly specific language, with a cultural background, semantics, syntax, spelling etc. that are very different from the Slavic languages. This is also why computer translations via Google Translate between Czech and Polish or Croatian are totally unusable, often even absurd and ridiculous. English is simply too different from the Slavic languages to serve as a pivot language or an auxiliary language on Slavic territory.
Of course, Interslavic is not the ultimate solution to these problems. Understanding it will always require some effort and a certain level of intelligence on the part of the reader or listener. But Interslavic at least makes it possible to communicate on a basic level with monolingual people in any Slavic-speaking country. And if you use Interslavic in writing, you can make yourself understood to readers in many different countries, no matter whether they know any other language beside their own.
It has been argued by some that it is a shame if Slavs use English to communicate with other Slavs. Wouldn't Russian, the mother tongue of almost half of the Slavs and understood by almost half the other half, be a natural lingua franca for Slavic people?
Russian is well worth learning, but it is not exactly an easy language to master. Like all other Slavic languages, Russian has had its own separate development from the common ancestor of the Slavic languages. Its syntactic structure is considerably different from the other Slavic languages (for example, the verb „to have” is practically absent, the verb „to be” heavily reduced both in form and use), and it is full of words and idioms that cannot be found in any other Slavic language. Without prior knowledge, a West or South Slav is unlikely to understand Russian.
Apart from that, Russian has been imposed upon the countries of Central and Eastern Europe as a lingua franca for a long time, and the effect is that many people, both Slavs and non-Slavs, have developed a strong resistance against Russian, which they have come to associate with „the language of the oppressor”.
At last, would Russian really be the best solution for Polish-Croatian relations?
So how about a Simplified Russian, then?
Russian is a living and standardized language already. Sure, a simplified form of it is not unthinkable. One might for example get rid of all soft consonants, reduce the number of declensions and conjugations, and eliminate all irregularity. The ultimate result of such actions, however, will be nothing but Russian with mistakes, in other words: bad Russian. In mixed groups, native speakers of the language of communication are always in a more advantageous position than those whose knowledge of it is limited.
Besides, a simplified version of Russian would perhaps be easier to learn and use, but not necessarily easier to understand for those who don't know it, nor would it address any of the other issues.
Another frequently heard point is this: we have Esperanto, so what's the point of yet another artificial language?
Esperanto is a language intended for the whole world, and it has been designed to be as simple and culturally neutral as possible. Whether Esperanto has succeeded in achieving its goals as a global language is disputable, but it is an undeniable fact that Esperanto is by far the most successful artificial language ever. It has, however, one major flaw: no matter how simple it is, one has to know its grammar and its lexical building blocks pretty well before one can understand it. The direct consequence is that knowledge of Esperanto is useful in communication with other Esperantists only, which essentially makes it a community language.
Interslavic, on the other hand, was neither designed to be used on a global scale, nor to serve as the language of a community. As far as it can be considered an artificial language at all, it is a so-called zonal constructed language, an auxiliary language for communication with or among speakers of a relatively heterogeneous family of languages, in this case the Slavic languages. It was designed in such way that speakers of these languages can understand it without even knowing what language they are dealing with. Although Interslavic grammar is admittedly a lot more complex than Esperanto’s, it is extremely easy for speakers of Slavic languages: all they need to know is basically a few simple tricks for modifying their own languages.
In other words, the benefits of learning Interslavic are of a completely different order than those of learning Esperanto.
Before Interslavic was (re)started in 2006 under the name „Slovianski”, there had been dozens of other Slavic-based language projects. By then, the older ones were obsolete and forgotten, while the more recent ones were underdeveloped one-man projects – with one notorious exception: Slovio, a fairly well-known project with a complete grammar and a large dictionary. Some have asked: why start a new project if there is already one such language around?
Indeed, Slovio and Interslavic are both Slavic-based auxiliary languages, but that is where the similarity ends. Slovio is the self-proclaimed language of radical pan-Slavism, yet it also claims to be „universalju”, a language for the whole world. Whatever be its true purpose, it is well worth mentioning that Slovio is not very Slavic at all: its grammar is almost entirely based on Esperanto, its word building mechanisms are mostly Germanic, and its vocabulary, although clearly dominated by (often mutilated) Russian words, does not relate in any predictable way to the Slavic languages either. As a result, its educational value is practically zero. Because of its artificial and un-Slavic character, Slovio has never succeeded in gaining any acceptation among Slavs. In addition, all Slovio activity has practically died out in 2011.
Unlike Slovio, Interslavic is entirely based on common Slavic material. It is neither intended to become a world language, nor to serve as the language of a speaker community, nor to propagate any political ideology. It is merely a tool for communication and education, not a purpose in itself.
Sadly, Slovio's creator has deliberately been spreading false information about other Interslavic projects and even resorted to promoting Slovio under their names. To avoid confusion, it should be emphasised that Interslavic and Slovio are completely unrelated projects. See the disclaimer for a short explanation.
Proto-Slavic exists only as a scientific extrapolation, but Old Church Slavonic and its various regional offshoots constitute a well-attested language – very similar to Proto-Slavic – that has been used as a written language for Slavs of different nationalities up to the 16th century. It has been argued that this would still make it an excellent candidate for a neutral Slavic umbrella language.
In a way, that is precisely what we are doing. It should be remembered, however, that Old Church Slavonic phonology, orthography, grammar and syntax are not only complicated, but also extremely archaic. Much of its vocabulary is no longer used in the contemporary languages, while words for modern concepts are lacking. To get an idea, just try to imagine what happens if you try using Classical Latin to converse with speakers of French or Italian!
To be suitable for modern communication, Old Church Slavonic requires a thorough modernisation, which entails a lot more than just adding words for „airplane” and „television”. Every single element must be held against the prism of the modern Slavic languages and updated accordingly. Each of these modern languages is the product of its own, individual development, which explains why words look differently from one language to another and sometimes also have different meanings.
To ensure consistency, we never borrow words directly from these languages. Instead, we compare their various shapes, return to their (Old Slavonic or reconstructed) their base forms and take those as a starting point for establishing the largest common denominator according to fixed, majority-based patterns. Thus we create a close approximation of the hypothetical language that might have emerged if the Slavic languages would not have fallen apart into an entire family of languages – or, if you prefer, of what Old Church Slavonic would have become had it been allowed to develop naturally over the centuries.
In other words, Interslavic begins where Old Church Slavonic ends, and can be treated as a modern continuation of it. One could say that the making of Interslavic is essentially the unmaking of developments that made the Slavic languages drift apart.