I think this can be generalized to "How to reach underserved markets?" In general the primary difficulties is that there are not well established paths for getting customers, and so that it is difficult to see where to start.
It is also worth noting for non-linguistically inclined readers that Slavic languages in general are very close to eachother which means that someone who is capable of translating Russian and Polish into English can probably translate Serbian as well into English. However translating out of a language and translating into the same language are very different so this is only a partial solution.
On the other hand, these can be seen as actual advantages. Customers in underserved markets tend to lack expectations regarding how to find vendors as well, so the lack of established paths means that you get to create your own path.
Like all markets, however, typically the major issue for getting customers is presence. This means you want to make a name for yourself, even if you have to do this before getting customers. Additionally you should look into what areas of markets are underserved.
For example, it may be far easier to find Serbian to English translation services than English to Serbian, because Slavic languages are close enough that a Russian translator team may be able to reasonably translate from Serbian to another well-known language but if they try to go the other way, it will most likely end up in Russian rather than Serbian (linguistics is a hobby of mine and I recently found that I could read Catalan based on my understanding of other Romance languages, but don't expect me to write it!).
If you can find partially substituting markets (such as Russian -> English translation services) you can look into how people find jobs there (because this is where the customers are likely to have to go for some jobs), and leverage those resources to find jobs closer to your core need. This can be an important resource for building a name for yourself and building up your own dedicated clientele.