How did cameroon become french spoken country?

That part of Africa was colonized by French explorers who were interested in exploiting the Natural Resources there. The natives spoke various tribal languages (still do to this day), but French was spoken as the language of the region that was included in the colony/country. The same holds true for all West African countries which speak French today. Each adopted the French language when they were being governed as a French colony, and now it is the common language, allowing people from various tribal/ethnic groups to have one common language.