Slovenia is sometimes considered to be part of the Balkans, but it is increasingly being included in tour packages as part of Central Europe, such as Austria, Hungary, the Czech Republic and Poland. Slovenians consider themselves Central European too due to their long ties to Austria. This is probably also correct as the Balkan peninsula is, according to some definitions, what lies south of the rivers Sava and Danube from the city of Belgrade. This definition however then would rule out part of northern Serbia, known as Vojvodina, a good part of Croatia, Romania and of course Slovenia. Historically all these countries/regions were usually considered to be part of the Balkan countries, as Slovenia too was once part of Yugoslavia. It is their recent independence that has allowed for the Slovenes to re-proclaim their desire to be considered Central European! Lastly, much of Greece also resides upon the Balkan Peninsula, but its major travel destinations lie upon islands.
Many of the countries in the former Yugoslavia (Bosnia, Croatia, Kosovo, Macedonia, Montenegro, Serbia, Slovenia) and Bulgaria are members of the South Slavic language group - therefore due to much commonality between words and language structure it is possible to communicate verbally between countries if you have a basic understanding of Bosnian, Croatian, Serbian, Macedonian or Bulgarian.
Some other useful languages might be Turkish, which many people in Greece and Bulgaria speak, and Gypsy may be useful in all of the Balkan states. Most of the people, especially in cities and touristic areas speak English. And sometimes German, Italian, French(in Romania).
There are numerous international airports in the Balkans. The major airports in the region are (by country):