Yoruba is one of the major languages of Nigeria, and many speakers have also emigrated to the UK, Brazil, and the US. It is written with the normal English alphabet plus a few extra "dotted" letters: Ẹ/ẹ, Ọ/ọ, Ṣ/ṣ. Also tone marks are required, grave ` for low and acute ´ for high. Lucida Grande is probably the best font to use.
The dotted letters can be made with the US Extended keyboard layout, using Option + x, followed by the base letter. Tones can then be added using Option + Shift + ` and Option + Shift + e. You can also download a custom Yoruba keylayout from my iDisk. This puts the dotted letters on the 1, 2, and 3 keys and all the vowels are deadkeys. You type the vowel and then choose a) 4 for grave accent/low tone, b) 5 for acute accent/high tone, and c) any other key for no accent/mid tone. The Naira symbol is on the = key. This layout also has the special characters needed for Hausa and Igbo (Ɓ ɓ Ɗ ɗ Ƙ ƙ Ƴ ƴ Ṅ ṅ Ị ị Ụ ụ).