According to data from Ethnalogue: Languages of the World there are at least 26 languages spoken in Botswana. The official languages are English, and Tswana – 70% of the population speak Setswana. Many people are at least bilingual, speaking English and their mother tongue, and possibly Setswana, or Afrikaans, depending on what is locally required to get work.
It's interesting to note here that all the world's languages have been grouped into around 20 linguistic families. Of these, four are very different from the rest. All these four are African families – and they include the Khoisan and Niger-Congo (the Bantu languages). The Khoisan languages are amongst the world's most complex languages; this is part of the evidence that has led linguists to believe that human language evolved in Africa, and probably amongst the ancestors of the Khoisan.
For more scientifically minded readers, here's a list of most of Botswana's main language groups, with their linguistic family roots and a few brief notes on where they're spoken. This tells you the history of that language, starting with the main group to which it belongs, and defining it more specifically. Thus English and Afrikaans are both Indo-European languages, and more specifically Germanic and West. That means they both originally derive from one ancient language, West Germanic. The classification of the Khoisan and Bantu languages gives a similar view of their history and relationships. Obviously information below is best read in conjunction with the previous section on social groups.